Hi mate,
All the information you need regarding your query is here:
http://www.ukba.homeoffice.gov.uk/settlement/
You can apply 28 days before so its really up to you. If i was you i would apply as soon as i am eligible to, so that if there is any more documents needed or the application is incomplete i will have time to sort it out 🙂
Hope this was helpful.
vimal