TL:DR - How long did your kid stay in a toddler bed? Or how long could they?

Trying to make DD1's room work for two kids is proving to be tricky. It's a big room but not a ton of empty wall space (windows, closets, etc.). We're thinking of moving her to a toddler bed instead of a twin to buy us some time, with the hopes that when LO2 needs to go from crib to toddler, we'll have the space to move DD1 to a twin. DD1 will be 3 1/2 in December when LO2 arrives.