When do you stop needing it?
When your kids can consistently do all the walking that will be needed.
When you can manage the touring plan to give short legs break when they need it (I struggled with that one for our
Disneyland trips and finally had to start taking solo trips to get my "commando" type touring out of my system).
When no one is falling asleep so hard on the buses or in the car that they have to be carried to the room...unless you have an adult in your group that can *easily*, and without hurting themselves, carry them.
That's when you're ready to drop the stroller.
DS still needed a stroller at DLR at 5... We brought the volo, even though his head is waaaaaay over the top of the back of it, just in case, for our WDW trip in December. He was 6. We never used it. Yay! I was really surprised, as he has a tendency to the same ghastly leg aches I am prone to, and they show up around 1am usually after he walks a lot, but no leg aches happened!
Since you are planning to rent one anyway, I'd just delay the rental for them. See how they do. If one or both show they need it...rent it. If they don't, then don't.