I am far from an expert on when to re-pot your Hoya, and I have lost many a plant after re-potting. Although there is no more vulnerable a time period for your Hoya than after re-potting, sometimes it just has to be undertaken. Some people root prune their plants and put them back in the same pot -I am not one of them. If undertaking this approach, one must also take some of the growth off from the plant itself, or the remaining roots will not support all of the vegetative growth. I like to try to get the largest possible specimen sized plant, and to do this most Hoyas need to be re-potted at least every two years, and some nearly every year. There are some exceptions to this rule and most of the carnosa clan would fall into this category. They can live and bloom happily in the same pot for years.
There is some controversy involving whether to always use a plastic pot or is it OK to ever use a clay pot. I will tell you that I use both. They each have their advantages. Plastic is light weight, and it is easier to tell when to water a plant in plastic by hefting the plant. Unfortunately, the plastic pot’s light weight is also works against it by allowing the plant to easily topple over. Do not use terracotta pots without some kind of a glaze – the pots lose moisture much too quickly and will soon kill the fine outer roots. I have successfully used terracotta pots by lining the interior with a plastic bag, and punching holes in the bottom of the bag for drainage. One other admonition about pot type - Do not use pots that are wider in the middle than on the top. These vase-type pots must only be removed by smashing the pot with a hammer if you do not want to destroy your plant during the re-potting process!
If you notice that your plant just dries out much too quickly, tip the plant out of its pot and examine the roots. If they completely fill the pot, it is time to move up to a larger pot. I like to loosen up the old roots by shaking some of them loose, or hitting them with a high pressure water spray. This gets rid of some of the old soil and loosens up the old roots so that they will more easily grow into the new larger pot. It is best to only move up in pot size 1 or 2 inches at a time. Transplanting into too large of a pot is almost certain death! After re-potting soak the plant well, and do not water again until you are sure that the new mix has almost dried out. I have rotted the roots off of countless plants during that time period. The most critical time to watch for overwatering is the first two months after the transplant. I have included a couple of photos oftypical Hoyas that are ready to be potted up. Good luck and watch the watering!