I don't know what the right answer is, but I know my answers ... I tend not to use someone as a reference if I haven't seen her in the past year.
Regarding gaps, I've twice had big gaps in my reference history. The last time it happened, it was for a year and a half, and it was because I'd met my mythical "the one", and wasn't interested in sessioning with anyone else (at the time, I only did FBSM once every 6-8 weeks, so it wasn't like an obsession. Much.). It lasted over 18 months, and then she retired, and that's when I realized: Oh shit, I'm fucked. Bummer because, at the time, there were no no-ref-required providers I was interested in. Luckily, I had sessioned at one of the groups, and that group still had me listed in their customer list, so I was able to pick out someone else from that group. These days, even if I decide I want to mostly see one person, I occasionally branch out to keep my references fresh.
WhatthefuckwasItalkingabout? Oh yeah, there are perfectly legit reasons for gaps in reference histories -- but if I were advising you, the longer the gap, the more I'd ask myself whether I were really comfortable either seeing or giving a reference