As for the month, the last week of May is Spring Bank Holiday in the UK, the schools are closed for 1 week and places are busier. Personally we would go for June, the daylight lasts longer (June 21st/22nd being our longest day of the year). The Welsh weather is unpredictable all year round - snowing at the moment!!
As for the length of time to spend in Wales, after reading your French travelogues you seem to cover a lot during a day, therefore perhaps 7 days or so, unless of course you wish to spend a couple nights in one area.
Hot air balloons in Wales, looking in our local telephone directory there is a company near Swansea and another in the Wye Valley, no web site addresses unfortunately, it's 11.55pm here so unable to telephone them at the moment. We are Christmas present shopping tomorrow (Tuesday) so we will call them on Wednesday). We assume it's the company in Swansea that pass just over our garden during the Summer months and we have seen them landing in the field adjacent to us. Btw our nephew proposed to his now wife in a hot air balloon but that was in England. Keep safe Linda &Geoff Wales