I spent $900000+ on a cocaine factory, the supplies are full but the production is taking so long, it's been about two days and the production has only increased by $20000, is that normal or am I doing something wrong ?
This FAQ board explains that production (and making your money back) will take a very long time, so I don't think you have done anything wrong.
This is likely because once you have delivered the supplies, you don't actually have to do anything else except wait for the money to come in. If the cocaine production was faster, it would dwarf the rewards from other events like CEO and VIP work.
As such, if you're short on cash and the production is taking too long, consider doing some of these other missions which have a quicker payoff.