How far in advance of applying to grad school can I ask for a reference letter?
I think if you're seven years out, the reference letters won't be as relevant. I would check with the master's program, but sometimes if you've worked for a certain amount of time you can provide one academic reference and one work related reference. I would try to keep a relationship with at least one professor - keep them updated, visit for coffee, etc. but wouldn't use a letter from seven years ago to apply for a Masters program. Maybe through your diplomas which will be more recent, you will find a more recent reference.