When should I replace my MH bulbs?

[ACBlinky]

I'm using 2 x 400W 10K SE XM bulbs on my 90g, along with 2 x 54W HO actinics, and I'm just wondering when they'll need to be replaced. I hear people say once a year, every 18 months, but here's my dilemma: I only have the MH on for 4 hours per day. I have primarily softies, with a few LPS, and everything seems to do well with the T5 on for 8 hours (2-10pm, MH for 4 (4-8pm).

Is the number of times the bulb is fired up the determining factor, or the actual running time, or is it a combination of both?

TIA

 

[JetCat USA]

they'll last you about 18 months give or take. the length per day isn't as big an issue on the bulbs as the on/off cycle. your 4 hrs/day isn't going to lengthen the longevity more then a few months.

 

[Entropy]

That is some serious lighting over a 90g with softies

You might want to think about down grading your lighting unless you plan to switch to all SPS, and even then I think 250w would be just as good (and use 300w less power).

Anyway I go 18 months on my halides at 8 hours a day unless I find a great deal or something.