Would it be possible for the Time Since Event to be compared to a variable in addition to the explicit hours:minutes:seconds already provided? It doesn't look the %some-variable% syntax works anymore in (most places), since RM5.1 provides the (much cleaner) drop-down for choosing variables everywhere except text fields. (But that option does not seem to exist for the Time Since Event condition.)
On a much smaller note, the only comparison "option" is "exceeds". For completeness, "does not exceed" might be nice, although that can be worked around with more complicated conditional/rule logic.
My apologies for making so many posts on the new RM5.1 features—I've been experimenting with them to see what they are capable of, and sometimes poking into corner cases. @bravenel don't get me wrong: I'm really liking the Sticky Triggers and Time Since Event in RM 5.1—for me these are the biggest improvements in RM since the big upgrade from RM4 to RM5.