Friend or customer? If for a friend, he supplies the beverages and he comes to your place and you both work on it and get it done.....
If for a customer, charge normal hourly shop rates for the job as long as it takes, minus of course any "unusual" stuff that takes a little extra time to deal with like an old part that doesn't want to cooperate and come loose like it should. In other words, if you have a fitting or something that won't come loose and you have to fiddle with it for an extra 30 minutes, don't add that time in. If it had come off like it should have and it went smoothly, that is the time to bill the customer for.....
Thouroughly confused now???
lol