POA isn’t a fixed time. It can be extended or shortened and just asking what time they reckon a truck will be ready will cover it and if its ready before that crack on and if its not ready you can extend it. Likewise prior knowledge also counts, in this case you’ve said yourself how long it takes for something to be ready.
Best bet if you like the client is to ask them how they want it done, work out if that way suits you and go from there.
Different for me as I’m full time and (now, finally!) get paid all hours regardless, so my want me to show some other work at the start, then it’s POA until you get a job if theres a delay, then as normal from then on. They are also part of that earned recognition thingy and they’d have been audited for that by DVSA so they must be fine with it.