Staying in EV mode for the longest possible time, is really hard, and annoying, because the car (seems) to turn the ICE on at different accelerations, depending on the state of charge of the battery. So, if the ICE is charged up full, you can accelerate decently (up to 42mph, if you're careful, and flat). But as the battery charge goes down, it gets progressively harder to keep the ICE from turning on.

But with that said, if the reason for your question is to get the best mileage, then there is A LOT more to it. Simply running in EV for the longest you can, with the resulting recharge happening later, will not be the most efficient (unless you just happen to get a long downhill, just before the ICE kicks on to start recharging).

If it's mileage you want, it depends on the driving that you'll be doing. I'm guessing from your question that we're not talking about highway driving. 3 big things that have worked for me, is getting a scangauge, inflating the tires, and pulse-and-glide.

The scangauge will tell you actual numbers, so that you can know if what you're doing is working. Inflating the tires up to at least maximum helps hugely.

Pulse&glide works best on roads where you can do about 40mph, and when you don't have people behind you. It's where you accelerate up to 42 (the highest speed at which the ICE can stay off), and get off the pedal. Then put a VERY slight pressure on the pedal, so that the battery is not getting or giving any current - like you're in neutral. And coast for as long as you can stand it, slowly going down to some other speed. Then accelerating up to 42, and starting all over again. Done correctly, you should be able to keep the battery charged just by accelerating. I've been able to do 49.5mpg on one trip using this technique (on New England hilly/twisty/busy roads). And I've had a couple 42mpg tanks.

Using this technique, the acceleration is NOT slow. It is decent acceleration. I know it doesn't seem intuitive, but it works. It keeps the ICE off longer, but keeps the engine warm, and the cat hot enough, and etc... So, quick jumps up to 42, then easy coasts down to whatever.

Good luck,
-Gren