Download the forms and look at the language, but I recall that they have to say how long they've known you. I think a general 3 years would be fine. I know what it's like, we tend to worry over every single detail, but, really, they're not, imho, so concerned about this with the citizenship forms. The IND is more fussy about granting an ILR, since there's monetary implications (ie. the dole) while there aren't any with citizenship. The biggest hassle was trying to reconstruct my entry and exits over the last 5 years. In the end, I later realized that I left out a 3 month period when I was back in the states. It didn't matter.