Everybody has reserves, to cover planned or unplanned outages. How much you fly on reserve depends on who you're flying for, your base and eqipment. Last year sitting reserve in DAY for PSA I logged one day of reserve flying for .8 hours in Feb, five days for 25.5 hours in March, and eight days for 29.6 hours in April. In the CLT base over the summer I averaged about 12 days and 55 hours. I jumped ship to Eagle and sitting reserve in LGA I flew 13 days for 60 hours last month. (I would've flown more but I sat in ATL for 3 days waiting on MX to fix an airplane) This month I've already flown 10 days and logged 46 hours. We're so short I expect I'll fly every day I'm on duty this month & break guarantee.
I sat reserve at PSA for 18 months, and there was no end in sight. Eagle want to hire 600 more pilots this year, so holding a line by summer is not an unreasonable expectation. I have a buddy at Continental who was one of the first hired two years ago, he sat reserve in EWR one month, then held a line. Transfered to IAH in his 4th month and held a line by his 6th.
USAir guys with 18 years are sitting reserve. (Because 17 year USAir guys are furloghed.)
Sitting reserve can be a good deal - you're getting paid guarantee and may stay home the whole month. Commuting to sit reserve is a form of punisment that should be restricted to child abusers and corrupt politicians.