Handy way to assess the spin time changes you can expect from a given change in either release or critical speed in a given top:
Δt = t2 - t1 = T1/e ln(ω1 / ω2)
where (t1,ω1) and (t2, ω2) are any 2 points on a purely exponential spin decay curve (SDC) such that t2 ≥ t1, and ω1 ≥ ω2. The "lifetime" T1/e is the time needed for the top to lose 63.2% percent of any given starting speed.
NB: All times should be in seconds, but the speeds can be in either rad/s or RPM as long as they have the same units.
First measure any 2 (time,speed) points (t1,ω1) and (t2, ω2) for the top of interest. Then rearrange the formula to determine its lifetime T1/e. Then do the what-ifs with the formula as written above.
Case 1: Your current release speed is ω2, but you hope to increase it to ω1.
Case 2: Your current critical speed is ω1, but you hope to reduce it to ω2.
Note that when you double release speed or halve critical speed, ω1 / ω2 = 2, and ln(ω1 / ω2) = ln 2 = 0.693. So either way, you'll extend spin time by 0.693 T1/e seconds. For example, if T1/e = 1,000 s (on the low side for a classic Simonelli), then doubling release speed or halving critical speed will add 693 s.
If you'd rather think in half-lives instead of lifetimes, the conversion is
T1/2 = T1/e ln 2 = 0.693 T1/e