limit the number of conversation one can create

In a web environment, I can design button/link so the conversation can be ended gracefully. But the end user may just close the window/tab instead of clickong my buttons/link, or click other links. I could end up with many unused conversations. This consumes memory. I can set a short timeout, but I will get Conversation Expired exception frequently. In reality, the end user may in two or three conversations at maximum simultaneously by opening 2 or 3 windows. So if I can limit the number of conversation one can open up, I can give conversation a long timeout. If I reach the limit, the oldest conversation can be removed. The oldest one is most likely the one not used any more.