I've only worked in large corporate settings as a software engineer and I found it pretty common to work maybe 75% of the time and the remaining I'm either relaxing or browsing around in open source communities. I don't think it's all too bad and a lot of my co-workers do the same where we bounce back and forth ideas during the 25% of "our time". I *know* it's common for a lot of engineers to work on open source projects on the clock but is it taboo to share this with future employers?
Is it bad for me to truthfully tell a future employer that I dedicate 1-2hrs/day to open source on the clock?
Depends how good you are and how valuable to the team you are. If you are valuable and you get more done in your 75% time that the rest of the team gets done in their 100% time, you should be ok, otherwise, you might find yourself in trouble. Maybe use your off work hours on the open source stuff, or only do open source stuff that directly correlates to your job so you can call it "on the job learning".