I do not have experience with Pentasa, it is usually not the top choice medication for UC, it is used more for Chron's. I do have experience with Asacol which is similar and prescribed quite often for UC. 500mg 8x a day is 4000mg which is not the max dose of mesalamine but it's a high dose which can put you in remission if you respond to the treatment. some of the users here take 4800mg of Asacol a day ongoing for years.
When I first started Asacol, I was started on 3600mg a day and it stopped my moderate flare up in just a couple weeks. I started feeling better pretty quickly and eventually all my symptoms improved and I was in remission for a little over a year. The flare while on 3600mg Asacol was extremely difficult to treat. I had to take prednisone and start 6mp. I don't think mesalamine is as effective for me as it used to be.