How close to 100% do you think the gains will stick long term 1+ years?
Depending on what you do, as much as all of it may be lost afterwards. Prolly no more than 20%.
Doing one of these programs is like doing a cycle of steroids and then coming off it. With or without PCT, you'll lose a large amount of what you gained from the gear. Your baseline will be higher afterwards but it may only be a difference of 10 or 20 pounds in the long run. (Gear works the same way, which is why most users keep using something; they don't only do 1 cycle and then never hop on a bike again.)
Smolov is a peaking program. The Bulgarian Method can also be used to peak. A peaking program is designed to get you to be your absolute strongest on the day of competition. Both of these programs derive the majority of your gains from neural adaptation: squatting heavy weight more often (3-4+ x a week for high-volume sets or multiple singles up to a daily max) makes your nervous system better at squatting; that is to say, they make you better at recruiting more of the muscle you already have.
These programs may not make you much bigger or induce any hypertrophy at all.
The higher your leg volume was before starting, the less likely you are to see increases in size.
Smolov is not designed to be ran in consecutive cycles and there is a large degree of diminishing returns for doing so, assuming you physically can. TBM will have you approach diminishing returns as you outkick your recovery coverage from continuing to tax your CNS without a break. These programs are for "blasts" not for "crusing".
Anyone ITT intending to run one of these programs who is
*not* competing in a full power powerlifting meet or gym meet (i.e. 1RM squat, bench, and deadlift all done in the same day) who doesn't realize the above is deluding themselves somewhat.