Nearly indefinitely from what I've read, but don't quote me on that. I'm sure there'll be someone more knowledgeable that I coming along soon...
you can keep cloning it over and over and over forever. a clone of a mother plant is the exact same plant
yep! long as you want. in one of the jorge cervantes movies he goes to a guys' garden who has been cloning the same strain for 9 years, he started with a single plant
ive personally seen quality degrade over 4 or 5 generations. after 3 years this guys bud looked great but sucked ass. you can however keep a mother plant around for years and keep cloning off of it and you will always have 1st gen clones. you wont notice anything if you do it a few times though but after doing it for a few years you remember the bud being better in the beginning