Now i`m pretty much gated to about 14-16 lights per day ( expert roulette, hard roulette and trial roulette) so it will take about 1 year to get my 3 Nexuses.
This seems to suggest that you only get light from the roulettes, or only once per day, but you can run as many dungeons as you would like per day and get light rewarded for every one. Except for time, there is no limit to how much light you can get.