If this is an application people will use more than twice in any given (short time period x 2) without changing these notifications will become extremely annoying. It's easy to think little reminders are helpful, but in reality unless they are very rare and only for significant events, they're just plain annoying. Remember Clippy? Never forget Clippy; "helpful" turns into "I HATE THAT $#%^ING PAPERCLIP" in only a couple of failed attempts at "helpful" actions.
Additionally, short term memory's temporal length is not well defined and varies significantly (from 30 seconds to 24 hours depending on situation and definition). Whether you remember depends very very much on whether you focus on/repeat/generally try to remember whatever information is in short term memory. And a switch back to the context may immediately jar "lost" memory--or it may not. You can't depend on timing here.
So instead of using a notification and/or basing this on how long they've been away from the computer, make it easy to find contextual information. Leave the current working context clearly labeled in the same place on each page, and make it easy to get back to the "default" view (if it's a common action) like this:
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A simple navigational element like a bar at the top of the screen can provide all the information you need to keep your place (and you don't always have to be idle for 30 minutes to forget where you are). Windows Explorer is a great example of this:
I get back to my computer, I can easily see where I left off. There's no chance your timing is off, since the element is always there; you don't need to guess at when I'll have forgotten the context. And perhaps even more importantly, the navigational aspect means it's useful even if I haven't forgotten what time I'm in, maybe I just want to go back to select a different time/directory/context.