I assume you're asking about open-jaws and long layovers?

On a round trip award, United allows:

• 1 stopover - defined as a layover of > 24 hours. This can be as long as you want, even months, provided you conclude your itinerary within 1 year.
• 2 open jaws - this is particularly helpful for Europe as you can easily take trains between places. An open jaw allows you to fly into one city and out of another. There's no advantage to using an open jaw on the US side, even though United allows it, unless it's to make availability work.
• any layover you have can be up to 24 hours