I was 20 years out of UG, 10 out of my PhD program when I needed LoRs. I had a very good relationship with my UG advisor, and although I hadn't spoken with him in many years, and he'd retired, he wrote me a fantastic LoR. I also asked my grad mentor, and I expect (hope
) he wrote a good one as well. I still have one left, as my dear friend, who is a very successful attorney, has yet to produce. It's probably okay, as I'll be seeking one from an attorney with whom I'll be working this coming year.
From what I've been told, by friends who are law profs and my admission consultant, a mix of academic and work LoRs is preferable if you've been out in the work force for a while. Thus, you probably will want to hunt down someone who remembers you from undergrad.