I have been using Elite TTX emulsion for a while doing intricate halftones successfully. About 4 months ago, I can't hold a halftone worth anything no matter what I do. Fresh screens, old screens, less time, more time doesn't matter. I use 2 - 500wt lights for exposing. I changed those bulbs at one point, with no success. I was wondering if the Ulano 925WR diazo emulsion would hold halftones better.
I've been reading here about exposing. I tried exposing higher,(done it before) with the TTX but I used a much gentler spray to wash it out. I had better results on some intricate halftone on a 230 screen. Large design. I go through a lot of emulsion, so shelf life shouldn't be a problem. I plan to order 1 qt at a time and save that emulsion for fine lines and halftones. I originally got it for discharge, but have not done much of that. TTX just acts as if it has had some "improvements". You know how companies are always changing stuff.
to answer your question....
I would burn between 1 1/2 min. up to 4, depending on when I coated the screens, 1 and 1. TTX you know is a fast acting emulsion. The usual wash out, though I have tried most all variations, I would wash with moderate spray, fan type on the front until I saw the image good, then wet the back enough to soften it. Go back to the front until it washed out. On very fine lines or light halftones, the image doesn't wash out completley or at all. Then the areas that did wash out would get too soft too quick. It would go from hard emulsion to blowing the stencil. Going to a gentler spray helped here, but still have to watch timing. I did one today 3 minutes and it did good, but took a while to wash out. Was getting testy at the end of washing. I did another one with very fine lines for 2 min. and I blew the first attempt at the second screen. Up it to 2 1/2 min on the second and rubbed the back of the wet screen at the very first. It washed out okay, but couldn't take much more.
"I did one today 3 minutes and it did good, but took a while to wash out."
Considering your EXP UNIT wattage and the TTX, the 2-4 minute window is about right, so you may want to run an image density test.
If you can cut some little strips of black electrical tape(or something equal in opacity) and place them to one corner of your next film, see if they wash out easier and faster after exposure, before the "hardended" areas get a chance to soften from excessive washout.
Also, make sure that stray, indirect light is not slightly pre-exposing screens, as it can mimic the issues you're dealing with.
FYI- there is an emulsion called Photocure Blu (Murakami) that fits the criteria you've mentioned so far.
I've been printing with the 500 watt units for 5 years now. I did not know about the glass issue, but I had removed them anyway. It is amazing how everything in the world can screw up a job. But the fine lines I am talking about are hard to hold. Just got done with a process job and almost pulled myself bald. Thanks to all. I am learning so much, even now. Just want to better myself and my work.
Diazo emulsion for halftone screens 
