We went on Tuesday this week (for the first time so not sure whats its usually like) and it took us about 2 hrs to drive round. We didn't hurry and stopped whenever we could. Although it was drizzley it was still warm and in the area where you have to keep the windows closed it was quite uncomfortable in the car. I'd say that if you only have a 1 yr old I'd wait a while before you go but if you have older children as well and the 1 year old doesn't mind sitting in the car for a couple of hours or is likely to be asleep it would probably be OK. We didn't do the rides area so I don't know how busy that gets. By about 3.30 the safari bit was much quieter and we went round a second time. They have an offer on where you get to go a second time for free before November (has to be the same car registration) so you could try it now and then go again later in the year. The boxs of animal food are quite expensive (£3 for a smallish box) but it was worth it as my DSs really enjoyed being able to feed the animals.
The PIL took DS (2) yesterday, and its not too bad if you get there early in the day. As its so slow to go round the reserves we sit DS on the front passengers lap whilst in there so that he can see out.