I've done several bits of it over various weekends/weeks - adding up to about 300 odd miles. Usually in stints of 5 - 8 miles of the kids are with us or 20-30 miles if they aren't.
I will say though that some sections of it are HARD. Properly hard, like it's only a few miles but it takes all day and there is nowhere to get off the path hard. The section called the Undercliffe that leads into Lyme Regis is supposed to be harder than the trek to base camp at Everest. Certainly we ended up crawling over, under and through fallen trees.
DH and I once booked out a whole week to walk the Penzance to St Ives section (camping so carrying all we needed).
We pushed too hard on the first day after the first section to Lamorna was really easy... we decided to head on to Porthcurno... oh my god. That section was hellish. It took us 2 hours to travel half a mile as it was all steep climbs over jagged sharp rocks.
This meant that I had threw a hissy fit on the morning of the third day when we woke up in Sennen and we decided to get a taxi to the next campsite in Levant and just stay there for the rest of the week - walking out as we fancied.
If you look on the website for the South West Cost Path you will see that they have divided the path into doable sections.
Good luck - it's a fabulous place! You'll walk past my favourite place on the world, Botallack, and past the best little holiday cottage ever, Wheal Call.
Gratuitous SWCP pics attached.