The Sabreliner takes about 3-4 seconds to deploy the reversers. Once they are out, they take about 3 more seconds to spool. Honestly, this is the first aircraft that I have flown with a clamshell type reverser and find them relatively poor. The brakes are much more effective. Speaking only from a pilots perspective, a cascading reverser is MUCH more effective (with a clamshell you are limited to around 70-75 percent N1 and they will burn the paint off the pylon if left out too long, but the cascading reverser gives you 100% N1 and you can use them to slow the airplane while taxiing. The amount of power you can use depends on the stow time vs. the spool down time of the engine.)
The positive thing is that Clamshells come out everytime and never when you don't want them too. Not the same with a corporate jet with a cascading system.
Take Care!