When I started I would leave it a month and hope, then I fitted a airlock, however I had to use an electrical stuffing gland to get it air tight enough to be sure it had finished, so started with the hydrometer, this however is also not 100% as different brews finish at different readings, so I moved to PET (POP) bottles, this also makes bottling quicker only 12 bottles for 40 pints.
I then moved to temperature control, and with that each brew is near enough the same, so set it to start and I know in 21 days ready to bottle, but that comes down to experience, before temperature control I did make errors and have needed to pour all the bottles back into the fermenter for a week where I have bottles too early, of all methods the hydrometer is likely the best, and I still check before bottling, however what ever time it says on the tin, double it.
There is no harm leaving the brew in the fermenter for 4 weeks, and if you are just starting I would do just that, then use old pop bottles so you can feel outside of bottle to see if too much pressure, plus release it if there is, only half a tea spoon of sugar required to prime bottles even when only using 12, I also have one extra just in case it will not fit, but 24 litres = 42 pints so it should fit in 12 x 2 litre bottles. But keep them out of sun light.
Once you find you like home brew, then you have to decide what stage your going to, I don't have the room for doing it all, so use kits, two can kits are better, but one can still near enough, I don't brew where I now live, so want it as automated as I can get it, so brew fridge/freezer and I know I need to visit 4 times in all to make a brew, once to start it, once to move from freezer to fridge, once to move to clean fermenter, and once to bottle. I also know time scale, takes 21 days in all, but this is because the temperature is carefully controlled, 6 - 10 days at 19 degs, then same at 24 degs, without that control you need to monitor daily, or give if far more time to be on safe side.