It's a bit of trial and error, actually. But a good starting time for 1:100 is somewhere around 60 minutes.

You should agitate a little at the beginning, like gently for the first minute or so, just to get everything evenly started.
Keep in mind that standing development is not fail-safe. It may go wrong with uneven development to follow.

What you really want from stand developing is the compensating action. Longer developing time with reduced agitation helps you develop the shadows more, while keeping your highlights in check. If you have time to experiment with one sacrificial roll of film before you develop anything important, then try using 1+50, but agitate only every 5 minutes. That gives you the best of both worlds with regard to normal agitation (avoiding uneven development), but getting the benefit in shadows from prolonged development time.

Whatever method you pick, there are no guarantees, unless you do a little trial and error first.