It depends on how long he/she has been unresponsive to your statement of picking up the requests. Personally, I give them three tries: The first initial pick up, after no response I post a second time reminding them I'll build, and a final "last call"
After the last call I go ahead and pick up another request just to keep things moving. Sometimes the person will return and be like, "oh yeah. I'm still interested, please build it!" and other times they'll never come back.