The subscription is still 5 minutes, but the topic itself you don't want to delete if you do not have any updates at all from SignalR. I guess if you get a new update after 12 hours and there's no topic anymore SignalR won't work unless you create a new instance or reboot?

Maybe it is a bit risky to have an IdleTopicTimeout at all. SignalR might crash if there's no updates before just after 1 day?