I've never had any issue getting the meal whenever I've wanted it.

A few days ago flying EDI-EWR I was asleep almost as soon as we were airborne - before they had even bought around the menus and taken orders. About 5 hours into the flight I went up and asked to have breakfast. They said they were about 30 mins off serving the pre-arrival meal, but when I said I'd prefer the breakfast that they served just after take-off they were more than happy to comply!

I'd say close to half my flights I either have the 'fast" option (when they bring everything at once), or have the meal at a later time in the flight. In fact, one of the things I like most about United is that their quick service option includes the main, where on most other airlines I've flown it's everything except the main course.