Yes I have walked Chesil Beach and for me it was around five and a half hours. I enjoyed it actually, but it is a tough walk and to be honest a rather featureless one. Within a little over 5 minutes walk from Ferry Bridge I saw no one until I was close to Abbotsbury, not even fisherman. It is really nothing but the sound of the sea on the shingle and a few gulls for company which I quite liked. It is very hard work walking on all that single though, so I suspect in effort terms it will feel a lot longer than the actual distance, despite the lack of hills.
To create a bit of variety I tended to alternate between walking just back from the waters edge at sea level and then along the top of the ridge.
Guaging progress is hard, so I used a GPS to give me an idea of how far I had gone (and had to go). Sadly, there is a vast amount of litter along the beach too (mostly washed up by the sea), as I guess the remote location makes it difficult to clear.
There are a couple of things to be aware of though. If the army range at Tidmoor/Chickerell is in use, it extends along the beach too and obviously on the beach you have no alternative but to turn back. This didn't happen to me, but it would be worth checking if the range is in use first. There is a hut at the start of the range on the beach with a flag. I assume when it's in use someone mans the hut and puts up the red flags. Also make sure you have plenty to drink, as there are no facilities on the way, as you would expect.
The other issue is that most of the beach is closed to the public during the bird nesting season, for a few months of the year. I assume this is spring time. The months during which you should not walk on the beach are posted on a notice certainly at the Abbotsbury end, probably also at the Portland end, but I can't remember what it said, sorry.
I'd advise if starting from Portland end, ask at the visitor centre first to make sure, they should be able to advise the months it is closed (or give them a ring).
Good luck, and I hope you enjoy it. It's a unique place and I found it a good and memorable experience.
The other part of the Dorset (and Devon) coast path I found a bit unsatisfactory was west of Lyme Regis, through the undercliff, to Seaton. An interesting landscape for sure, but not really what I considered coastal - just the odd glimpse of the sea through trees for many miles. I understand this path has been closed now anyway, with a diversion even further inland. I did manage to walk the whole way along the foreshore (at low tide), but again it is tough with lots of rocks and boulders to get around, although much of it is beach (a mixture of sand and shingle). Loads of fossills to be seen on the rocks, where they are largely untouched by the masses, due to the difficult access. If doing this I'd avise starting at Lyme. The sea was closest to the back of the beach at around Seven Rock Point, although it might have changed now, due to the fast coastal erosion around there.
I have photos I took on the walk of Chesil beach here :
https://www.flickr.com/search/?tags=chesilbeach&sort=relevance&user_id=70332320%40N00
