an element of the logic is missing: which condition starts the calculation of elapsed time? You might want to look at [clocker] and [==] or [&&], and perhaps post an example patch demonstrating the source of values and conditions to test for.
If what you're trying to ask is "if $f1 is < x now, and remains less than x for the next 100 msec, then do something" the basic way to go about it is to start a [del 100] at now, and if, at any time in the next 100 msecs the condition breaks, then send a 'stop' message to the [del] object, cancelling the scheduled event.
This is the standard technique from the "Time-Based Event Handling 101" lecture notes. Useful one to have up your sleeve, even if it's not what you're after right now.