Citation Encore takes about 4 seconds to deploy enough that some reverse thrust is noted at low speeds (taxiing), and about 2 seconds to deploy enough that aerodynamic braking occurs at high speeds (landing). Either way, it takes about 4-5 seconds before the "clam-shell" doors have fully deployed and additional thrust can be added.
Don't know about how much they decrease the landing distance, but have seen regarding the following corrections to landing distance figures with the TRs inop (these come from the CRJ performance manuals and factory approved procedures):
one or both TR(s) inop: increase required distance by factor of 1.10 (10%) for landing and 1.15 (15%) for takeoffs.
Thrust reverser inop operations limited to Dry/Damp (non-contaminated) runways only.
Example:
Performance charts for TRs operative indicate landing distance of 3500 feet.
If TR's are inoperative, multiply distance by 1.10 for an inop distance of 3850'.