i think its different form individual to individual...itachi had his mangekyou for over ten years and started to lose sight only before his death, while sasuke already almost lost his vision within a few days....
i guess its a part of the answer is the fact that kakashi had the sharingan implanted as well as the fact that he didnt use kamui only after shippuuden... we saw him use it against deidara 2 times. then against pain 2 times.. and at the current war several times...(kunai,Rasengan,Narutos KB, attempt to kamui gedo mazo head, hachibi, himself... madaras defense. obito....i think thats it