If you have a 51% forever you can refuse to chain the blocks that contain the unwanted transaction. Every time it's added to a block you just fork the chain, and as you have the 51% hashing power your fork will eventually win.
The time to crush the other fork depends on how much hashing power you have. With 60% you will generate 60 more blocks in a day that the other fork, in average. With a 51% you will generate 6 more blocks per day. With a 50.1% you will generate only 1/2 extra block per day, so you need a few days to win.
The time to crush the other fork depends on how much hashing power you have. With 60% you will generate 60 more blocks in a day that the other fork, in average. With a 51% you will generate 6 more blocks per day. With a 50.1% you will generate only 1/2 extra block per day, so you need a few days to win.