i had to drive an hour to get to my private scan at 7+4 weeks. i only drank about 500ml but silly me, started drinking at the start of the journey and by the time we were half way there i was fit to burst, and the potholes in the road REALLY didnt help. thankfully i got seen right away, and she said to me - oh my god thats your bladder, its huge you must be very uncomfortable lol.
i'd say half hour before, amount is a hard one, depends how much other fluid you've drank recently. i wouldnt have been able to drink any more than 500ml