We can guess the distance between some islands is not very big…
They reach Whiskey Peak the same day they enter Grand Line and meet Crocus and Laboon. Nami is wearing the same clothes. They leave the next morning.
Nami was stung by a mosquito on Little Garden and was examined by Kureha on Drum Island three days later. They leave Drum the same day at night.
Before they reach Drum, Vivi says she's not sure they can reach Alabasta in a week, which means the distance between Drum and Alabasta isn't a lot more than a week. Not sure how many days they spend in Alabasta, but I guess it could be more or less measured.
Robin shows herself to the SH right after they leave Alabasta - she didn't hide several days, right? A few minutes after she officially joins, the ship falls from the sky and they meet Masira. Masira's territory is around Jaya, so they're not too far away from the island - besides, all but Robin wear the same clothes on Jaya as they did when meeting Masira. So, I guess it's the same day or at most the day after. They leave Jaya the next morning.
The SH get to Skypiea a few hours after leaving Jaya. The day after, the survival begins. So, they don't stay long in Skypiea either.
When they go down to the blue sea, they meet three sea monkeys. Those creatures are still around when they see the Frog Captain's crew. They all wear the same clothes between the moment the Frog Captain's boat is sunk by the sea monkeys and the time they reach Long Ring Long Land. They only stay one day on the island.
The distance between Whiskey Peak and Little Garden , and the one between Long Ring Long Land and Water Seven, are unknown, but if it's about the same as the other islands, then we can estimate the time spent in Grand Line between 3 and 6 weeks.