As @Hardrock_keto_chick said, it’s all up to you how you do things. And yes, any time-frame that you put between meals, is always beneficial. So going from Dinner to say lunch the next day, usually falls around 18 hrs. give or take, so 15 hrs. is good indeed. … For me, I’ve always said the ‘not snacking’ between meals is one the best benefit you get from Keto, but you will also find that after some time (different for everyone) you simply find you’re not as hungry anymore, (part of becoming Fat adapted) and this is where some folks like to start really look into fasting.
As for coffee, shouldn’t hurt at all. If you read above, you see a lot of folks do what they feel is best for them, and we are all different. So there’s no set rules per se, but by simply trying different things to see what works best.