Chester's a full day for me. On my own I can get around everything once in 4 hours or so if I were pushed for time - generally I take the full opening hours over it.
To see Pairi Daiza properly definitely needs more time than any British zoo. Only the Berlins really compare. And actually, I'd much rather do the Berlins in a day each than attempt PD. PD has both lots and lots of animals and lots of space/gardens/walking to see them - probably more a Tierpark Berlin than a Zoo Berlin - but it's even more of a maze, it's not as well signed and there is constant visual stimulus and distraction everywhere ("Hey, that 8ft tall massive sculpture just sat by the wall has been done to look like it's real jade! Oh, it is real jade..." "What's that rock propped up on old bit of farm kit there for? Oh, apparently it's the world's largest opal..." "What's in there?" "A real, giant seaplane").
On our most recent trip we did most of the zoo the first day, with the exceptions of the Chinese gardens area, birds of prey and the then-small North America section. We did these on the second day, plus the elephant bath, train, bird show and repeating some of the more interesting sections.
Both the Aquarium and the Reptile House Boat Thing are deceptively large. And in both places there are also enclosures in the gents to watch for!
It's a two-day zoo to see it fully for pretty much anyone with a zoo nerd's interest levels now. If you want to do it in actual relaxed manner it'd probably need three. That makes it pricey - €35 a day plus food etc adds up - but there is genuinely nowhere like it. I still don't know exactly how much I'd say I really like it - for its scale it ought to be popping up in top 10s and things but it doesn't quite feel like it sits among the 'normal' zoos in that way. But it's something every zoo nerd able to should see!
I just came back from here... Its all the things you said it was, weird, wonderful, bizarrre- all these things are Paira Daiza!!! Like a cross between an eclectic and high quality zoo and Disneyworld.

I had one whole day and one half day there. I saw nearly everything I wanted to in that time but I could have spent more time still quite easily. I missed the aquarium and reptiles though (not my particular interests anyway) and didn't discover that the Elephants had a much larger paddock and lake at the back of the house, until I noticed this on the map back in the UK!
Among all the bizzarre temples, the superb japanese gardens, boat restaurant, lighthouse etc the quality of the visual presentation of the animal exhibits is extremely good I think- far superior to most UK zoos with their limited viewing areas etc- that's never an issue at PD. The chinese/asian area and collection in particular I thought was particularly outstanding.
The high spots for me were the Spix macaws, which I have wanted to see for longer than I can remember, those both these and (to a lesser extent) the Lear's next door still don't show very well, clinging on the wire high up a lot of the time. Other species I haven't seen for many years- or never, included the Giant Pandas, Golden takins, Blue sheep, Tassie Devils, Crane sp. Shoebills, Bulwer Pheasants (now moved into the Oasis house) but I could go on and on. Overall impression- there is nothing else like it is probably all I can say...

Also one other point- for people thinking the extended summer hours are helpful, when its open till late into the evening - I think you would be too exhausted to spend that many hours in the day walking around and still looking at animals! More for evening eating I think... But the additions this year and next of the Polar and Northern areas(currently all just a massive building site) will definitely extend the time needed to see everything properly even further into the 2 or even 3 day time period!