Because Harker's story of his visit to Count Dracula comes to us as a diary, we have an exact account of how long he stayed. Harker arrived on May 3, and Count Dracula allowed him to leave on June 30th, a date very close to an earlier date Dracula had mentioned of June 29. Dracula says to Harker on June 29:

To-morrow, my friend, we must part. You return to your beautiful England, I to some work which may have such an end that we may never meet.

Dracula essentially keeps Harker a prisoner in his castle until that date. Harker has grown increasingly uneasy and then terrified about the mysterious count who only shows up at night, never eats, and seems to be able to turn himself into a bat. Harker writes a letter trying to arrange transport out of the isolated castle, but Dracula intercepts and burns it. Harker becomes convinced he will be killed, so he is relieved when he is freed.