I don't think they have established exactly how long, this show isn't good with details behind such technicalities.
When Damon came back to the tomb where Katherine was to gloat about how he knew about killing an Original it looked like Katherine was already unconscious due to lack of blood, but at the same time she said to Stefan when he was in there that dessication is excruciating so maybe she hadn't passed that barrier yet.
That angry dude from the tomb vampires mentioned something to Damon about the pain he felt when he was down there, he said something about weeks or months but I'm not sure.
When Stefan tricked Damon in season 1 he said something about it's good that Damon exhausted his energy because that would mean his heart pumps faster, making the process quicker.
I think it's implied to that due to the effcient weakening effect vervain has injesting it in high doses increases the dessication process but that's my speculation on the matter.
"FREDERICK: 145 years left starving in a tomb, thanks to Katherine's infatuation with you and your brother. First few weeks, every single nerve in your body screams with fire. The kind of pain that can drive a person mad."
Amara had been desiccating for 2000 years and became a statue like figure. But it took only minutes for her to thaw out as did Silas drinking Jeremy.
"DAMON: Where is my ring? STEFAN: Won't be needing it anymore. DAMON: How long have I been here? STEFAN: Three days. DAMON: What are you doing? STEFAN: During the dark ages when a vampire's actions threatened to expose or bring harm upon the entire race, they would face judgment. They sought to reeducate them rather than to punish them. DAMON: You know what will happen if I don't... Feed on blood. STEFAN: You'll grow weaker and weaker. Eventually you won't be able to move or speak. In a week your skin will desiccate, and you'll mummify. A living corpse. Unable to hurt anyone. Ever. DAMON: So what, you're just going to leave me in the basement, forever? STEFAN: I've injected you with enough vervain to keep you weak. Once your circulation stops, I'll move you to the family crypt, and then in 50 years, we can reevaluate. DAMON: I'm stronger than you think. STEFAN: You always have been. But you're not stronger than the vervain. And we both know it. I'm sorry. Didn't have to be this way."
"STEFAN: Keep it up, Damon. The more energy you expend, the faster you'll go."
Stefan and Damon talking about his desiccation after he almost kills Caroline in season one You're Undead To Me.
So apparently we can determine the following: The skin will desiccate within 2-3 maybe 4 weeks. The more energy they use the faster this happens. Then mummification like Silas and Amara eventually a living statue. In the originals when Marcel puts them in the basement they are like moaning zombies, like the tomb when we first get in Pearl looks like the crypt keeper but that was 145 years and they could still open their eyes and move a little.
Wes had perfected a drug that he used on Enzo that caused him to desiccate within a short period of time and shut him down. What is in that is a mystery and what would be the long term ramifications.
Kat seemed to last for ages when shut in the tomb without doing much other than being a bit pale, but that could just be her character and that she was not vervained or using her energy so it was going slow as possible.
What do you think?