Eek, you don't really want a 24/7 supply of a psychostimulant, this'd be an easy ticket into the padded room / stim psychosis from not sleeping.. with a steady release of meth you wouldn't be able to sleep for too long.. but it's a good question, I thought similar about whether one could put e.g. isopropylphenidate into a slow released for a la lisdexamphetamine / vyvanse (which isn't exactly an extended/slow release, they just made sure no one gets a rush from it, but it seems to last slightly longer than plain amphetamine). Realize I don't even know how they make the time release with depot injections, guess some polymerization or whatever high tech magic.
Just know that some poor people die off depot injections releasing too much too fast, this isn't seen often but this risk exists.
I'd like a methoxetamine low-dose time release injection. Besides that I hate needles that is.