It depends how large your supply hose to the boiler is, and the CFM you pump into it. You can charge it pretty quick if you really wanted to (not sure why anyone would) but it's a good technique for drying everything out when winterizing an engine. I've moved engines around without a tender like this on level track and you could probably run it about 1/4 mile, but as was mentioned it depends how tight your rings are. I'd bet that with just a boiler full of air, a tight engine could make it a good 20% farther than an identical one that was worn.
This is also a good illustration as to how important it is to maintain an engine for fuel efficiency, and how much power the steam has stored vs. air.
John Hillier