One oestrogen test just gives the amount at that particular point in time and the levels vary day to day and according to time of day. Also in relation to when and where you apply or take your HRT.
The levels you need depend on why you are taking it and your age to some extent. If you are under the menopausal average age then best to have it higher. If much older then the level required to eliminate symptoms, so it depends how you feel. If for osteoporosis prevention then I think around 200 would be the minimum ( there was a paper that suggested somewhere just under this level but that was some time ago and sorry haven't got it to hand.).
Another medical article I read somewhere suggested that levels should ideally be the approx average level across the menstural cycle as it would be throughout the month. This of course is variable, but the article suggested around 300 pmol/l would be a rule of thumb.
I've had my levels measured a few times and since being on HRT they have never been as high as 300 pmol/l and probably the low 200s. A year or so ago they were measured at patch change and were just under 100 I think and the patch dose was increased although at my age (mid 60's) not sure it's really a good idea.
So how long is a piece of string and depends on the answers to some of those points!
Hurdity x