In "my" outfit standbys are planned in blocks from 1 to 5 days depending on how many pilots per fleet are based at that airport. Longer blocks for more pilots. If a four of five day block is planned two mandatory off days have to follow it. Standby means that the pilot is contactable up to 16 hours a day, between two consecutive standby duties at least eight hours of rest/sleep time has to be rostered in which the company cannot contact the pilot. Reaction time is one hour from callout until the start of the duty. Each standby duty has a number next to it (S1 to S6) which denotes the maximum length of duty that can be done out of that standby, S1 means that the pilot has to be back at base the same day, S5 at the end of the fifth day, S6 is special in that only longhaul duties longer than five days can be done out of that, not shorthaul or shorter duties, can only planned a few times per year. How big that figure can be is determined by the planned standard rotations that leave the base that day. If the longest rotation is just two days, no S3-6 can be planned.
Additionally there is reserve which means the pilot is contactable 24 hours, has to be home by 07:00 the following day. The duty has to start on the reserve day. Reaction time is six hour for this duty and only a limited number may be planned per year. Therefore the earliest duty can start at 06:00 and the latest call out can be done at 17:59.