The entire length of the South West Coast Path is 630 miles which is a pretty long walk.
For some people who walk at a fast enough pace they could complete the South West Coast Path within 30 to 40 days.
However you may want to stop and take breaks for a few days before continuing on down the path so it may take you 2 months especially if you're not a very fast walker.
The South West Coast Path has lots of towns and villages along the way to stay in so you can set your own pace and stay for awhile in those towns and villages when you get tired.
Some people have completed the south west coast path walk in 30 days while others take 40 to 45 days and sometimes as long as 60 to 70 days.
It all depends on how fast you walk, your fitness and how often you stop for a break.