Before making an offer - you should feel able to as many times as you need to be sure. Better to be sure when you offer than to just view once for 10 mins and offer and then decide a week later it isn't for you. If I was going for a 2nd viewing with a serious view to offering, I would make copious notes about things like where plug sockets were, and look very carefully at kitchen and bathrooms to see where work might be needed.n so in my view, 1st viewing is to see if you like the house in general terms - rooms size, layout, location etc and then the 2nd is confirming that info in your own mind plus a much more thorough assessment of the state of the property, so you know what you'll be getting.
After offering, I think really just once is normal. The process should be fairly far along before anyone thinks about buying stuff. Before you go, you should think carefully about all t he things you want to look at and measure, knowing that will be the only visit .
If someone asked to visit post-offer, I would say yes, but make it clear that I'd expect just 1 viewing like that - whe organising, ask them or ask EA to ask them to make sure they know everything they want to look for beforehand and perhaps leave a tape measure and paper out for them on arrival so everything can be noted down. If someone asked for a second such viewing post-offer and close to exchange, I wouldn't say no, because who wants to piss off the buyer over a 30 minute visit, but I wouldn't be thrilled and I probably wouldn't feel the need to have it pristine and viewing ready at that point.
As someone else says, viewing to do measurements close to exchange, if it's once is a good sign actually.
If someone had only viewed once, I'd worry a little that they had offered without being serious and might be more likely to pull out. Bad signs arising from later viewings are things like not being interested in doing measurements but going round and making negative comments about the fundamentals of the house such as room size or layout, whi hands the kind of things that might suggest they are u decided about the whole thing and might pull out.