This all depends on whether you have water damage on your tub walls. If so, then total replacement make sense. If not, bath fitter option can be considered.

Normally, it will take the professionals 2-3 days to gut and install the entire tub area (tub, faucet, walls). This is pretty involved since you are pretty much dealing with demolition, plumbing, boarding, tiling, and possibly a little taping. Also, you need to know suitable products and materials to give you the best value and quality to meet your budget. So, if you are not prepared for this kind of challenge, leave this kind of project to the Pro.

The bath fitter option I personally do not recommend it. There are actual cases that water leaks in between the tub liner and the old tub, and the tub bottom becomes very bouncy - the famous "water bed effect".