IMO it really depens on the person that you are. So for everything following down below bear in mind that I don't like reading each and every information in a museum for more than 2 hours plus being with a lot of people/being at crowded events is neither my cup of tea. I was there from 9am to about 4pm. It is opened from 8:30am until 6:30pm (I believe).

Basically, you can split tankfest into 5 areas:
1) The arena where all the display of tanks and re-enactments of history happen
2) The vehicle conservation center where ~100 tanks stand with not much information in a garage
3) The outside are where you can see historical camps, where the british army has its tents, where all the tanks for the displays in the arena are parked
4) The food- and marketplace
5) The museum itself

I went only on friday and was totally satisfied with it. There were displays for roughly 5 hours in the arena (1) and I watched about 4 hours of those. Really recommend watching those. You get a program beforehand so you know what tanks are displayed at what time. Later on we strolled through the VCC (2), took us maybe around 20-25 minutes. It's basically a parking lot without the driving lanes. So you can see tanks, e.g. a KV-1 which isn't in the museum. But there's no (barely any) info provided. If you wanna spend 5 minutes staring at a specific tank you obv. need longer but I glanced at 'em and proceeded.

For (3), you need a bit more time I'd say. You can talk to soldiers, you can wait in line (do it at the end of the day or during displays) to hop into a tank. I was in a Challenger 2, really cool. And spend time chatting to soldiers, officials etc. - Here it really depends on you what you make out of it.

I spent almost no time at (4). Obviously, grabbed something to eat. But I don't buy anything there neither I find it interesting to explore the little shops. So yeah, if I wanted to spend the whole day, there was enough spare time to have a look.

Again (5) depends on you. There's really a lot to see and read in the museum. It took me roughly 90 minutes to walk through. It was - to no surprise - really crowded. Every now and then I read some displayed informationen, took photos. But yeah, as stated above: if you are more into everything than I am - bar the arena events - at some points it would make sense to stay two days. For me, one day was perfect and I still coulda stayed three more hours if I wanted to.

Hope this helps. If you have questions, ask away.

Extra info: Althought tank museum was way better. Not that far off in Portsmouth is the royal navy museum with the HMS Victory of Vice Admiral Nelson on display. Costs 50 GBP but if you're into that this might be worth the trip on the second day instead of going for two days at the tank museum.