How many days was Juliet in the tomb before Romeo came?

Certainly less than two days, maybe less than one. She took the potion which was to make her look dead for 42 hours on Tuesday night. They found her Wednesday morning and then had to make arrangements for the funeral. After the funeral Balthazar has to ride to Mantua: when he gets there Romeo has just wakened, so it must be Thursday morning. Romeo buys the poison and returns to Verona at about the time the 42 hours are up, some time Thursday evening. The time from when Romeo gets the news to when he arrives at the tomb must be about the same as the time from the funeral to when Balthazar gets to Mantua.

Juliet must have taken the potion at midnight or later or she would not be waking up at night. The preparation for the funeral must have taken most of the day on Wednesday: if the funeral took place before six p.m., Balthazar would have got to Mantua so early that everyone, Romeo and Apothecary included, would be asleep.

Let's say Juliet took the potion at 2 a.m. Wednesday morning. She would wake up at 8 p.m. Thursday night. If she was buried at 8 p.m. Wednesday, Balthazar gets to Mantua at 8 a.m. and Romeo gets back to Verona just in time for her to wake. She would have been in the tomb 24 hours. With a little different numbers it could be a little more or less.