Their fertility and usability depends on so many factors that it would be hard to nail it down to specifics.
A microscope is almost essential in order to be able to evaluate the size of the pollen grains. Without that, you'll waste a lot of time and effort. I've had conversions that were more pollen-fertile than most of the other tetraploids I grew at the time. Homeward Bound was one of those. Others are just average, and some are not only pollen-fertile, but are actually fairly pod-fertile, although these tend to be rare.
Conversion is such an unpredictable undertaking that plants can be unaffected, "fully" converted, or partially converted, where the leaves on one side of the converted fan are thick and pebbly-textured, and the other side are relatively normal, or where a scape will have both diploid and tetraploid blooms. These sorts of things tend to even out one way or the other over time, but not always.
While converting a plant, some latent vegetative buds may be affected, and while the main fan will turn out to be diploid, a year or so later you might find scapes with tet pollen. I've bought treated diploids from people who thought their conversion had failed, and the next season had them turn out to be very usable conversions. I had one conversion which had foliage so thick and bumpy that the fans only got to about 3" tall for several years, and the leaves would break instead of bend. It finally bloomed about 5 years later, and the pollen was a damp mess, with very large grains. Jamie Gossard talks a little about polyploids in his description of Heavenly New Frontiers (2016)
http://www.heavenlygardens.com...
When a conversion increases, you can't assume that the new fans are tetraploid—it's best to check the pollen with a microscope and actually make viable seed with it before you stake your reputation on it. If, when checking under a microscope, the conversion shows a low percentage of over-sized grains, you can still make viable seed, but it will require a fertile pod parent and a lot of crosses.
