For artists with large/highly-engaged fanbases, we find that once a month is a good interval, and for artists with smaller fanbases, once every 2 or 3 months is ideal. That said, many artists play weekly or even daily.
One highly recommended idea is a routine – fans love a series. Here are some examples of artists doing really well on Concert Window:
Liz Longley doing a monthly "Live from the Living Room" with a special guest each time
Ellis Paul doing a monthly series of shows from his home
Seth Glier doing a special show roughly once each season
Artists are still finding new use cases for Concert Window, so let us know if you come up with something cool!