From page 71 of the R20 manual:
"A project can have a maximum of 1350 bars. Recording will stop automatically if it reaches this length."
The number of bars in your project depends on the tempo and time signature. For example, if the tempo is 120 BPM and the time signature is 4/4, then you'll use up 30 bars per minute (120 BPM divided by 4 beats per bar). Coincidentally, 1350 ÷ 30 = 45 minutes. So, at 120 BPM in 4/4 time, your maximum recording time is 45 minutes because at that point you'll have recorded 1350 bars which is the maximum that the R20 can record in a single project. Faster tempos will use up bars at a higher rate. For example, 240 BPM in 4/4 would use up 60 bars per minute, so 1350 bars would only last 22½ minutes. And slower tempos will use up bars at a lower rate, so you'll get longer recording times. For example, 60 BPM in 4/4 would use up 15 bars per minute, so 1350 bars would last 90 minutes.
If I were you, I'd check the total number of bars in your project each time that you stop playing between songs. To be on the safe side, I'd stop recording when the bar count gets close to 1,000, then start a new project for the next group of songs. Better still, just start a new project for each new song. Then you'll have no worries about reaching the maximum bar count.
Update (2022-09-01): Just thought of another possible solution. Since you want to record multiple songs, presumably of varying tempos and time signatures, then you probably don't care what the project's tempo and time signature are set to. So when you create a new project to record your band's practice session, set the project's tempo to 40 BPM (the minimum value allowed) and the time signature to 6/8. Since there will be 6 beats to each bar and 40 beats each minute, you'll use only 6⅔ bars per minute. So you'll be able to record for 3 hours and 22½ minutes before you hit the 1350 bars limit.
Update (2022-09-08): In theory
, the maximum recording time should be 3 hours, 22½ minutes with the tempo at 40 BPM and a time signature of 6/8. However, I just checked this on my R20 and found that the maximum recording time in 3/4 and 6/8 is exactly the same. So you'll get the longest recording times in 4/4. You can find the maximum recording time on the R20 by scrolling to bar 1350 and switching to the Level Meter View Screen to see the time in minutes and seconds. Here are the maximum recording times for the minimum tempo (40 BPM), an average tempo (120 BPM), and the maximum tempo (250 BPM).
Maximum Recording Time (min:sec)
As you can see, the longest recording time possible on the R20 is 135 minutes (2 hours, 15 minutes) which you can achieve by setting the tempo to minimum (40 BPM) and the time signature to 4/4.