When you say you are exchanging with another boater, does that simply mean that you are hiking out and he is hiking in, or do you physically need to meet on the Bright Angel bridge and exchange passwords and the secret handshake?
Last summer we had a large exchange, 8 out and 8 in. Those hiking in left the rim at 6:30 and the first group arrived about 1:00 PM, but they were going slow knowing there were others behind them. Since it will be cooler I would say the range is between 5 -8 hours depending on how in shape they are. I have seen it done in less than 4 hours. I second the Pipe Springs exchange. If you need to get water etc. at Pantom, you can send one or two boats down early to pipe springs so that the person going out gets an early start and then the other boats can resupply the water and meet you down there. We camped at cremation and got to Pipe Springs at first light to give those going out as early start as possible.
You had me at 'Hey, Row'