Twice.
Once after I get the Coil(s) mounted.
And once Again after it is Wicked/Primed and I have put the RBA Cover back on.
A Classic Building Mistake is that a Coil Leg may not be clipped short enough. The Resistance will read fine when the RBA Cover is off.
But when the RBA Cover is put on, and the Coil Leg touches the RBA Cover, now you have a Dead Short.
I always do it four times. Once after I get the coil(s) mounted. A second time after I worked out all the hot spots and I re-tightened the post screws. A third time after I finalized the positioning of the coil(s) and I dry fired the coil(s) once more, to re-verify the stability of the (fully glowing) coil(s). And a fourth time after it is wicked/primed and I have put the RBA cover back on.
That said, care needs to be taken when positioning the coil(s), as the coil(s) tend(s) to move very slowly out of position while vaping, if the coil(s) is/are still relatively fresh. I routinely keep checking every once in a while during the first week or so, and, to avoid getting in trouble, I make sure there is enough (more than just 1mm) space between the coil(s) and the inside of the barrel, in cohort with the whole inside of the chuff (if applicable), and, whenever I'm using a metal drip tip, also the bottom edge of the drip tip as well. As for clipping the legs of the coils short enough, another thing to also want to consider is to ensure clean cuts so no pieces of metal can break off afterwards, and always remember to check for stray clippings that may lurk inside the build deck, everywhere, including between the posts and the side of the deck i.e. including in places that are too frequently overlooked.