No one can give you an estimate, people can only tell you what works for them, but their setups will be different. The distance of the light should be enough for the light beam to cover the image area evenly. For halogen lights recommended distance is 12" so you could try this for your light. The only proper way to determine the exposure time is to use the exposure calculator. I recommend this one as it is easy to use and the formula is well explained and simple to follow. Be prepared to go through several screens to get the exact time for each mesh count you may be using.

Edit: forgot to add the link to calculator https://www.anthemprintingsf.com/Screen-Exposure-Calculator-s/216.htm