PD projects are screened by at least two different individuals at DonorsChoose. They require price quotes for all expenditures listed, and actually the daily food amount is capped at $30/day. Although I haven't seen a published rule about the number of days that can be approved for a PD trip, I do know that the screeners request brochures or fliers from the conference or workshop and separate price quotes from hotels, airlines, and car rental places. I have a strong feeling that if the material submitted for the conference showed a 3-day PD, and the hotel quote was requesting 2 weeks stay, they would most likely flag the project and send it back to the teacher asking for justification for the discrepancy between the two. Actual humans look these projects over and make sure that they are legit before allowing them to "go live" on the site. For the food allowance, they generally approve it as a reimbursement and the teacher has to pay out of pocket and then submit receipts afterwards.