The rule is when they show interest in climbing out, it's time. You can do it before then, though. My daughter made the transition right at about two years old. She was a trooper about it. We talked up the big girl bed until she was used to the idea, then we brought it in. The first night she was pretty scared and I had to sit with her for a long time. Even then, she cried for a bit. But, once that first night was over, she took to it pretty well. She sleeps a LOT better in the big bed.
Interesting how it worked out that we switched her, by the way. My daughter never showed any interest in climbing out, but the day before we planned to make the switch, at naptime she stuck her leg through the rails and then fell sideways. It twisted her leg pretty bad and left a mark. She was lucky not to sprain something. That pretty much clinched it .. time for the big girl bed
BTW - I don't see the point of buying a toddler bed. It's just an extra bed to buy. There's no reason a kid can't go straight to a single bed. I think toddler beds are just a way for the furniture companies to make more money.