The two I've worked for allowed me to always hold a line, but occasionally I would have a reserve day. With the short staffing as most claim, I don't think you'll have to worry about being a reservist at many regionals even a year for now.

To answer your question, it probably depends on the airline and whether you are home based or a commuter. I was home based, so I would just sit around the house until I got a call to come to work. These periods were generally 4A-4P, at least for me. Other times, I would get a call the night before telling me that I was flying the next day so I would know in advance, especially if they had me dead heading somewhere to do a flight. For me, I liked the reserve schedule as none of our lines were over block and I rarely got called. These days, I can imagine it being more of a nightmare with the short staffing.