i don't see a reason not to do it whenever you want. yeah if your small you won't make much money out of it, but every little helps as they say, just because you have a small sub base does not mean you don't have a loyal following, and a few of them might love to give something back and help out the guy or girl that gives them hours of entertainment.
i think the big thing is to not shove it in their faces, yeah make a video saying you have one, and maybe mention it in the odd video, just don't push it
i also think its a good idea not to hold back content unless they are a patreon donator, i have seen many channels do this, and your just going to annoy people that can't or won't donate because your basically saying your taking time and effort and therefore videos away from your normal channel to make videos just for those that pay. thats pretty dickish imo
i plan to start a patreon at 5k subs, it might only get me $10 a month if i am lucky, but the way adsense is going, i might need that money!