My SIL did this at six weeks. Had been nursing. Didn't leave enough pumped milk, didn't care. My kids are two and four and I'd have a hard time leaving them with my mom (who lives with us) for more than 18 hours. Had to with dd (18mos)when ds was born (planned homebirth, midwife didn't believe I was in labor, she left and I delivered at the hospital a couple of hours later), but it was at dd's own home, and under extreme duress on my part. And for one night. A WEEK? I really wonder what goes through some people's heads when they consider their level of responsibility in raising the children they birthed. I think some people pump em out because it's the cool thing to do...without considering the level of personal input it should/will involve.
When my kids are seven or eight, I'll think about it seriously! Not that I don't trust my mom as a caregiver...they're simply attachment parented, and none of us would benefit from being apart that long.