Per Sylvania, these bulbs will not last as long as oe. Says so on the back on the package even. Especially on vehicles which use them as a drl and they are on ALL the time. I've got a set in my 04, not used as a drl, but a fair amount of that is night time driving.
Some things to check: is the housing sealed correctly? I'm assuming yes...
Is the bulb being touched when installed? Again assuming not...
Is the wiring connector melted/look like its getting hot?
Depending on how many hours you drive in 6 months, that could be about right. I put probably 30 minutes a day worth of night driving on my truck, 5 days a week. If that gives you any baseline. And they have been going strong for about 2 years.
Side note: When i worked retail in the auto parts business, we sold these bulbs along with all of the others sylvania offers. People came in fairly often with blown ultra/zxe bulbs after 6-8 months bitching about how a $50 set of bulbs didn't last and demanding warranty. EVERY time it was a vehicle which used them as a drl during the day. Every time. More often than not I would recommend a refund, and buying stock replacements. They just last longer. As for hid I have no experience. Sorry.
