If you're just using a tapewormer then I would do fecal egg counts 4 times a year, timed so you do one before your tapewormer in case you also need to use a combination wormer instead.
If you were planning on using a combination wormer like Pramox in spring and autumn then you need only do fecal egg counts summer and winter.
I would use Pramox as above as fecal egg counts don't pick up encysted redworm, which a straight tapewormer wouldn't deal with.
If your horse is at low risk of worm problems, which should be discussed, then I would advocate three tests per year plus Equest Pramox in winter. You test at roughly the 'quarters' of the year except for winter. If no eggs seen then you don't need to add other worming doses.
Other situations may require slightly different regimes, but there is lots of info on our website at Westgate Labs and I'm happy to help if you wish.
Also I will be at Your Horse Live this weekend with our trade stand, if you happen to be there so do say hello!