First, you don't need to have the VTT open in order for one of your players to use it. They can log in without you, if that's all you're trying to leave it open for. Second, you can leave the VTT open as long as you like, and it wouldn't really be any more of a resource hog than pretty much any other webpage (the resource use would increase when another player enters the VTT if you have incoming or outgoing video/voice). However, your session will expire after I believe 24 hours. If you refresh the page at least once every 24hrs there's no issue, but if you stay until your session expires you'll get a popup telling you so, and you won't be able to do anything until you dismiss the popup (which will refresh the page, giving you a new session). Of course, if you're in more than one campaign, whenever you refresh the page, it loads the VTT you most recently entered. So if you open one VTT and leave it open, then open a second campaign's VTT, refreshing the first VTT will load a second copy of the second campaign's VTT.