Filter with fast reset:
Feed Schmitt input via a resistor.
Add capacitor from Schmitt input to ground.
Optional - add diode across resistor, Cathode to Schmitt, Anode to voltage source (ie diode "arrow" points at Schmitt).
Diode adds "fast recovery" which simple RC lacks. If Vin is low 51% + of time for periods < RC time constant a simple RC will slowly discharge and trip. The diode "resets" the capacitor rapidly and means dropouts of ~= t = RC+ are needed to trigger output.
If Schmitt is already fed by a resistor divider just add cap + diode to input resistor.
When input goes high cap charges via diode to Vdiode < Vmax then cap tops up via Rin.
When input blips low cap starts to discharge via Rin with time constant R x C.
If Vin restores high then diode will allow charging faster than if just via Rin.
Time constant = Rin x Capacitor value.
eg if C=0.1 uF, R=10k. Tc = R.C = 10,000 x 10^-7 = 1mS = not long.
100k , 1 uF = 0.1s.
100k, 10 uF = 1 s
Cap best to be NOT electrolytic due to leakage but depends on cap and brand.
Actual delay till triggering depends on how far Vin is above Vtrip but 1+ timje constant usual.
As always, component types and an actual circuit diagram would help.