I've wanked 'till it bleeds' but never seriosusly injures myself stim wanking.
Imo if you dont want to spend the whole of the time you're consuming w/e aphrodisiac substance, either merely looking/searching for porn/or w/e wank material floats your boat, the answer is very simple; get the deed over and done with immediately you feel the effects. That way you will be freed from your desires and become master of your own actions and time. This will probably give you at least a few hours to do something different other than looking for/organising/or wanking to porn.
You may well get the urge again within a few hours, once again if you don't want to be a slave to your desires, just get the thing over and done with immediately.
I know myself how it's possible to spend so many hours on the 'build up' to a 'stim wank' but in all honesty you could be having much more fun doing something else instead; listening to music, even tidying up and other household chores.
Ultimatelty spending extended periods of time on a stim based wank session is such a depressing and futile activity, and if you do spend all those hours on the build up, when its all over you'll just feel empty, miserable, and depressed, pretty much imho.8(