I'm not sure 20-30 minutes is enough to gauge a session. At the Irish session I attend on Thursdays we don't get singing until the end of the evening, if at all, but singers are welcome and appreciated. At Irish sessions I used to attend in Holloway, the sort where there was a lock in, singers generally took over from the players towards the end of the evening as they were less likely to be heard by a passing policeman but also because the atmosphere by then was better for singers with the audience being quieter and more mellow. This didn't mean that the dance tunes stopped altogether but the players were appreciative of a chance to rest, drink and chat (quietly!) and would start again if the singing stopped and the landlord had been persuaded by his wife that the police weren't likely to bother us.