Probably 3 to 5 mins. You will need to experiment with this time as it depends upon the distance the LEDs are away from the board, type of UV leds used, current etc.
If you over expose UV gets through the mask and you end up with very porous/thin or no tracks. Its pretty obvious as soon as you develop the board.
Using the Sun.
The Sun's intensity will vary considerably so its not really a good 'constant' source like the LEDs.
However, the basic method would be to get a coated board and cover it with a thick card (mask) . Every two minutes Use a permanent marker to indicate the edge of the card on the board and then move the card down exposing a fresh 1 cm strip. When the board is completely uncovered you can develop it and see what time the strips were under exposed, correctly exposed and over exposed.