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I haven't used this jsandbox script, but it appears to have what you want.
If one script freezes on a page, other scripts will not continue to run. Therefore, there is no way to detect if another script has stopped running, without using a custom plugin or something. Browsers do not use multithreading in that way.
You could set a timeout in the main window which stops / deletes the script after 10 seconds. Then you just have to clear the timeout when the script has finished (just add a line like this to the iframe script: window.frames[0].clearTimeout(window.frames[0].timeoutName) -- I don't know if it works, but it should)
I think this would largely depend on the script and how browsers handle scripts in iframes.
Let's say there's a while(true) in the iframe.
The browser may either lock up, or crash the tab (like what Chrome does), or it might lock up the iframe. If it locks up or crashes the tab, there's nothing you can do with JS itself to prevent it, other than attempting static analysis on the script to find possibly problematic statements (Static analysis to find problematic scripts like that would never be foolproof)
Assuming the browser only locks up the iframe or does something else while still allowing the scripts in the main page to do things, removing the iframe after a certain period of time is an option.
The browser might also display the "Script is slow" popup. In this case, it will most likely either completely shut down all scripts in the entire tab or just in the iframe. If just the iframe, the other scripts in the tab could still clean up after it after the predefined period of time.
Another alternative would be to pre-evaluate the script in a separate runtime where you can detect things like that yourself. You could run the script in, say, Rhino, and determine if it takes too long to process, or something similar.
I don't know if this would work exactly, you might be able to get something like this to work with a little bit of tinkering. I take it that you are taking in text JavaScript and then evaling it, right? You could parse or maybe even just use regex to replace all of the for, for..in, while, and function declaration's call with the function call then some logic that calls your code and figures out if it has been running for 10 secs. If it has it will either return; or break; or something. The code would likely freak out afterward and probably start throwing errors, but at least the script would stop.