Re-arm has no effect on wep times,when you're out you're out.
In the 5 10 15 example you have 5 mins of wep then require a 10 min cool down time and you have a 15 min total so you can run wep 3 times at 5 mins and require 10 mins of cool down between each 5 min period. Total cool down times would be 30 mins 3x10 mins for cool down and 3x5mins for total wep.
Basically it takes 2 mins of cool down for every min of wep,up to a total of 15 mins of wep, in the example above.This does vary somewhat with the different capabilities of each plane. IIRC the KI84 requires 1 min of cool down for every 2 mins of wep. Although as I said each plane or series is slightly different. If you look at the link Fugi supplied you will see it explained in detail.
I hope that explains thing and is the answers you're looking for.
