There is no hard-and-fast rule for leaving your child alone for a few minutes. Before making a decision to do so, you need to carefully think about the overall safety of the child and your own comfort levels. It's usually fine to leave a young baby alone in her crib while you take a quick shower, for example, but this doesn't apply to swings and bouncy seats, which aren't as safe. (If you're really nervous, you can always tote baby in her car seat into the bathroom with you.) Once your child's around 2 or 3, she can probably safely play alone in a childproofed room for a few minutes (as long as you're within earshot, not across the street). While you're out of the room, listen for the normal babbles and squeals of your child playing. If things seem too quiet for too long, go back in to check on her and leave again if you need to.
Of course, this also depends on the room -- never leave a child alone somewhere that's not fully childproofed or with access to a pool or water. You should also never leave your child alone in the bathtub until he's at least 7 years old. (A kid who slips and hits his head could drown in as little as an inch of water in minutes.)