Depends on the fight, for me it has been 15 years (my case is unusual, involving several states and federal court).
The average with fighting is about 3 years.
If both parties are willing it can take it, anywhere for 1 month to 6 months. Or if the other party resists but doesn't really fight then 3 months to 1 year. *These are just estimates, each case is different and it depends on the courts you go to.
Also it depends on if it is multi-state. If you are both in state, the other party will have 30 days to respond, if in another state 60 days, if another country 90 days. If the other party is in another state it can double the length of the case. Also it depends on how long it takes to get a Summons on the other party (the get personal jurisdiction). Sound complicated? That why people hire lawyers.