That's a good bulb. Do you already have a T5 bulb and fixture that size? If so, then you may not need to replace it yet. A good T5 should last at least a year before needing replacement.
I have the bulb that same with (bulb was already in the fixture when I bought it at the store). The fixture says "USE ONLY 39 W T5 HO LAMP" and the actual bulb says "REPTISUN 5.0 UVB"
How long should I wait to replace it if I want to be on the safe side? I have always hear 6 months, and 5 to be safe... is that only for T8 or non-HO bulbs?
Yeah the T5 bulbs should last at least a year. I measure mine with a Solarmeter and my Arcadia T5 is still going strong after 3+ years. The 6 month recommendation comes mostly from T8s and older bulbs. Best to replace it after a year or so if you aren't measuring the output just to be on the safe side, because they do decay over time. The 5.0 bulb has a lower output than the 10.0 / 12% bulbs so you'll want it at about the same distance as a T8 (6-8'' above the basking area). I would replace it with a 10.0 or 12% bulb if you need to mount it further than that. You can still use the existing T5 fixture for those bulbs.