If you can keep to your word clients like the heads up-especially the elderly. Ball parking your time frame to a few hours is perfectly OK for mowing or light install work. When a large contract is involved and you have other work be upfront and let clients know you may be stopping by to start incremental parts of the work as your time allows and you may not have the time to notify them. Most clients will be on board with this as long as you have told them up front and memorialized it in your contract. Correct contract language can go a long way to mitigate any problems. Some clients will be way too picky about times-they are usually the same clients you will be "picky" about when it comes to renew for the next season.