Hi and welcome Seph! I was a rollie smoker for over 30 years and started on 18mg which did me fine. Everyone is different in how much they vape, but I guess at about 3 days on a 10ml bottle. To be honest though, I have several flavours on the go at all times, so it's hard to calculate.
An ego is a good starting point and I would recommend you get some Evod BCCs to go with it. They are great, don't leak and have replaceable atomiser parts. If you take to vaping long term, you can then learn to recoil them yourself, which saves money, but that's in the future!!
Good luck and there's loads of good info on here, I recommend the wiki pages (at the top) as a good place to start!
But if your going to be using a ego battery you would probably be better using a pro tank mini or the v2 version there a prefect fit for that style of battery