My fiancee and I have been living together (with proof) since March 2008. We have been apart for 5 weeks now while she visits family... is this too long of an interruption in our relationship?
After the first year of continuous co-habitation, you are in a common law partnership, and you will continue to be in that common law partnership until you live apart for a certain number of months due to a breakdown in your relationship.
Even if this had happened (which it hasn't in your case), you can resume the relationship after only one day of renewed co-habitation. In your case, there has been no breakdown, and you have co-habited for more than one year in the past, so you're fine. I suppose they could wonder if your relationship has broken down, but I don't think it will be hard to prove that it hasn't. There hasn't been an interruption to your relationship, only a temporary interruption to co-habitation.