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I personally think it doesn't sound good [me and my own opinion, of course]. If a session needs a manual termination button, there's a chance someone forgot to click the button, and thus, one step backward from privacy [as explained by examplens and bhadz]. If dev wants to venture into an option to manually terminate a session, I think it'll be better to reverse the proposed idea; instead of having the manual termination button by default, dev add a check mark to allow people to keep the session longer or until they pressed the termination button. Otherwise, the sessions will end like always.
Not sure about the implications behind this, though. Wouldn't it mean sinbad has to keep a log [albeit selectively] to cater this idea? I can imagine some people will not welcome this idea.
Yes, that's a good idea by having that feature and it's just like an expiration if ever the user forgots to do that and the devs don't have anything to do with it. The session can be ended on an instant based on the user's discretion and if not, it is automatically gone based on the set period of time.
I'll use this chance to re-emphasize that I personally think it's not a good idea. Having a possibility to manually terminate a session by preference implies that Sinbad keep a log of user's transaction. Be it due to the user's request and preference or not, it's a step back from the anonimity that supposed to be offered by a mixer. It'll be a very great assurance to anyone that the platform --any mixer platform-- does not keep a log longer than it needed to be [that is, until the transaction is finished].