You could trigger it, unless you're the type of purist who hates using triggers ever.
Say Buff lasts 30 seconds. Instead of making the behavior last 30 seconds in the modifications + in data, make it set a Real to 30 which, through triggers, is decreased by 1 every second. (In terms of data, make the behavior last forever.)
Then make a trigger that when Real = 0, remove behavior.
Then make another trigger that when Behavior changes (increases), increase Real by 30.
See the Refresh Stacks and Extend flags in Buff Flags
i got extend and refresh stacks flags on, but its not increasing duration time from each stack :/ and i cant use 1 stack one because each stack of my behavior decreases attack speed, movement speed and doing periodic damage effect from each stack. Well mixing trigger and data is not working for me asswell because I'm using visuals for debuff duration, its like indicator igame, and to change duration thru triggers i have to set maximum value in data, and when i decrease duration value thru triggers my visuals looks like half filled.