I would like to decide the length of the record time. So when I start recording it can go for n hours and then stop to avoid loading the database and agregating data for hours not that interesting.
I also wonder if there are any limit today, seems my install stop recording after 25 minutes.
there is no limit today. If your install stopped after 25 minutes, there must have been some kind of problem. Did you get an error message? You could check the log files (C:\Program Files\Jet Profiler for MySQL\logs). It should just go on and on (we've run it for days without stopping it).
However, the performance will degrade as more and more data is collected, since a lot of disk I/O is required when analyzing. What we are going to add is an auto-purge setting so you can have a rolling window of N hours. Any data older than that will automatically get deleted. Does this sound ok?