I started RTK vol 1 about a week ago, and I'm moving at a pretty fast pace, so I think I'll be through it in about 2-3 months. I'm wondering how hard it is to learn all the readings after that though? Does it take longer, or maybe its quicker?

I would say it takes significantly longer.

There's just more readings than there are kanji, because each kanji often has several readings. And then there are countless compounds for each kanji, and knowing how to read the kanji is only part of it; you have to learn the meaning and usage of each compound.

However you don't need to learn all the readings & vocab to start reading. By the time you learn readings for about 1000 kanji, with a good amount of vocab as well, you can make pretty good attempts to read many things.

or you could just apply the kanji to the vocabulary you learn instead of learning the readings separately... it seems a lot more simpler that way ^_^

To learn all the readings? Probably a lifetime and even more. I imagine there must be some rarely used readings and exceptions even natives don't know, in rarely used compounds. I'd say try to stop worrying about it and just learn words and sentences after (or during, if you have time for it) RTK. And have fun and try to read a few easy things (manga is a good place to start and actually *learn* readings because of the furigana).

yeh even if you did learn all the readings you would probably still come to kanji compounds and still don't know how to pronounce them because you are unsure which reading to use... learning kanji readings in context is the best after Heisig ^_^

It's not hard at all, it just takes time and patience. Just read and listen to as much Japanese as you can to learn new words.