Yeah, cook time should be the key variable. You may not be able to get all of the injection in, but put in as much as you can.
Give yourself plenty of time. General rule of thumb for butts is 90 minutes per poind at 225 - 240 pit temp (measured at the grate). Sometimes smaller cuts take longer per pound. It's better to have it done early rather than late. If it is done early you can always wrap it in a couple of layers of heavy duty foil or put it in a foil pan covered in foil and then stash it in a dry, pre-heated cooler with towels or clean newspaper for insulation. It will stay hot for several hours.
Don't forget to take pictures! Notes would also help so you can adjust things for the next cook if you need to.