Assuming a clean fret-board, I use a piece of cotton rag or even a paper towel as an applicator, apply just a few drops to the applicator, and then lightly rub a thin coat over the entire board. I find that a very dry board will soak up the oil quite fast, often in only 2-3 minutes and certainly in less than five minutes. If the board is dry after five minutes, I reapply the same way. I'll repeat this until the board still shows some oil after a few minutes. Once I get there, I wait 10-15 minutes and then wipe off any excess. Then I'll either put the old strings back on, or wait a few hours before playing it with new strings. I do this because I sometimes find that when I play the guitar right after oiling, the strings discolor.
Bill, tgo