It took me close to 3 years before I could really seriously let someone genuinely in my space and was free enough emotionally to seriously consider a future. But I am love addict, so I have a sickness. It takes long because I feel guilty not necessarily because I loved the person.
When we are young, yes, we can jump to a new relationship pretty fast, without considering it a rebound. When we grow older, no. Not me, at least. We become more invested, so the time needed to heal is longer.