No skill points are gained during the negative effect. They are gained once the negative effect (debuff icon) expired.
Per 1 second of negative effect duration you gain ~0.0045 points.
The amount and types of negative effects do not influence the points gain. The points gain solely depends on the negative effect duration.
Metabolism reduces the duration of negative effects, which makes long-lasting injectors and stimulants give less points but safer to use.
Max Energy has a minor 5-minute stress resistance debuff, so drinking one every 5 minutes is a cost-effective way to gain skill points.
Meldonin injector has multiple minor 15-minute debuffs, be sure to bring extra food & drinks as it drains your energy and hydration.
Obdolbos 2 cocktail injector has multiple 30-minute debuffs. It can be employed on longer raids. Be sure to bring extra medkits, as it causes you to lose health.