The Length of the Conversion Process

The Length of the Conversion Process
Often, it is impossible to determine exactly how much time elapsed between when people heard the gospel and when they responded. However, in a few instances the Scriptures tell us how long it took for someone to hear the gospel and respond to it:

Acts 2:41: the first Christians heard and responded to the gospel in a single day.
Acts 8:26-39: the conversion of the Ethiopian took place in a single day. (He seemed to have no prior information about Jesus.)
Acts 9:1-19: Saul (Paul) was first confronted by Jesus on the road to Damascus. After three days, Ananias arrives to tell him what he must do (Acts 9:6) and Saul is baptized.
Acts 10:1-48: Cornelius had a vision to send for Peter, and four days later (Acts 10:30) Peter and his entourage arrived and preached the gospel to him. That same day, he was baptized.
Acts 13:14: The Jews and Gentiles in Pisidian Antioch both responded positively to the gospel on the same day as having heard it (13:43, 13:48), though these were a week apart (13:44).
Acts 16:13:15: Lydia of Philippi seems to have heard and responded to the gospel in a single day.
Acts 16:25-33: The Philippians jailer responded to the gospel in the middle of the night "at that very hour."
Acts 17:1-4: The Thessalonians were persuaded over a period of three weeks.
Acts 17:10-12: The Bereans seemed to have believed after short period of examining the Scriptures and Paul's words daily.
Acts 17:16-34: The Athenians responded after a single instance of Paul preaching at the Areopagus.
Acts 26:28-29: Agrippa questions Paul whether he could become a Christian after such a short presentation. Paul's response allows that conversion could indeed take a short period of time.

The most rapid conversion was probably the Philippian jailer, the longest might well be the Thessalonians or Bereans. In the cases of Saul and Cornelius (where events span several days), it must be observed that the actual time of teaching and response was still a single day. (Of course, people like Saul may have been exposed to elements of the gospel for many years prior to their ultimate acceptance of it.)

Conversion Could Happen in a Day
While the length of time of conversions varied, the fact of conversion in a single day is prominent. It follows from this that the length of time for conversion was dependent upon the recipient, not something imposed inherently in the proclamation.

The reality of rapid conversion in the early church has several implications:

The gospel was presented succinctly.

Not only is this evident from the actual proclamations in Acts, the length of time for proclamation and response does not allow for extensive, mandatory pre-baptismal instruction.
Any conversions that required more time were because of people needing more information, study or reflection (e.g. Bereans, Jews in synagogues, etc).

There were no "tests" of suitability for baptism that would impose any time delay.

The only criteria seems to be faith in the gospel and the desire to be baptized and live faithfully according to the gospel. This commitment might well be expressed in the simple confession, "Jesus is Lord" (ref. 1 Corinthians 12:3, Romans 10:9, Philippians 2:11 etc.).
There was no time to "prove" repentance prior to baptism. Apparently, the convert's expression of desire to live faithfully was accepted by the apostles and early church.

What about Paul's remarks about "proving their repentance by their deeds" in Acts 26:20? This says nothing about pre-baptismal "proving." Paul's point was that people don't just get converted and continue living the same way as before, their conversion results in a change of their way of life. In fact, this was a frequent motivation given for Christians to address things in their lives (e.g. Romans 6:1-11, Ephesians 4:1, Colossians 3:1-11, et. al.).

There was no time for extensive examination of sinful behaviors such as "sin lists." Sinfulness was a given human condition (Romans 1:20) that could be forgiven in Christ.

The record of the early church indicates that much "primative repentance" from sin took take place after conversion. To name a few, there are the cases of Simon the Sorcerer (Acts 8), the Ephesian magicians (Acts 19) and numerous references to repentance in epistles (e.g. Romans 6:1-2, Colossians 3:1-9).

There was no time to "prove" religious commitment, such as daily habits of prayer and Bible study, church attendance, financial giving, having a learner's heart, evangelism, etc. Similarly, comprehension of various other spiritual truths were not a matter of conversion.
While Jesus urged those who would consider following him to "count the cost" (Luke 14:28) as a matter of prudence, the early church does not seem to have made this an explicit part of the conversion process. The church seems to have implicitly known the "costs" of faith in Jesus as the Christ and counted them of little worth compared to the promise of the gospel (e.g. Philippians 3:7).

Ultimately, brief conversion testifies to the simplicity of Christianity: Jesus is the Son of God, crucified and raised from the dead according to the foretelling of God. Those who believe this may respond and have the gift of eternal life. Yet, the Scriptures allow that conversion need not always be brief, as there is evidence that many converts took some period of time to examine the Scriptures prior to responding.