I will answer this pretty simply:
The duration of a seal is only as long as the amount of chakra placed into the seal. Lets take Naruto's seal for instance, during his fight with Pain he loses control and during his unconcious state he speaks with Minato via the chakra he left in the seal and gets Naruto back in control of his body by using his chakra to restore the seal on his chest that had worn off and deteriorate over time.
Now given that the Uzumaki seal is one of the strongest there is and granted how much Chakra Minato used to place it on Naruto the fact that it only lasted about 15 years should give us a good estimation on how long regular seals would last if alot of chakra is put into it.
But when considering the seals Minato places in battle one would expect the duration to be alot smaller, keep in mind he places them for the purpose of killing an enemy, and the more chakra he puts into forming it the less chakra he has to work with. With that I would wager that he only puts enough chakra in the seal to have it last 30minutes to an hour. Which is more than enough time to put down a tough enemy.
Even with that said you should remember that seals can be broken. So the theory that Minato just FTG's to Obito fails for 2 reasons. 1) The seal would have worn off by now. 2) If it hadnt worn off then Obito could have broken the seal.