Toddler bed question

We have a free toddler bed (from a family member, mattress included) that we're going to transition DD to before the new baby arrives.  I found a super cute bed set on sale at Target today but I'm wondering how long will she feasibly stay in a toddler bed?  I know most people don't spend the extra $ but since it's free and we don't have a twin bed for her yet we figured we'd give it a shot.  Plus, our bedrooms are small in the townhouse so this will give her more room to play - the other option was a double bed but we'd need railings and it would take up a lot of space.

Thanks!