How Long Does It Take to Fix a Medium-sized Teeth Gap?

your gap is not too bad... i would agree that you can clsoe it in 6-8 months and get a stable result. I would recommend traditional braces so your outcome will be more predictable and shorter.

 

Moderate gaps of the anterior teeth close rapidly.  It may take about three to six months to close a moderate space. The time of closure of teeth spacing also depends on the age of the patient and a few other factors.

Thank you for your question AND including the photo. Invisalign is definitely the way to go and, assuming we are taking care of the front teeth only, the treatment will take a very short time, no more then 6 months. An examination by a treating doctor should be done to make sure.
Like with any orthodontic treatment, a retainer should be worn for 22 hours following 6 months and at night afterwards.

Regular braces and Invisalign can work equally well for correction of space between front teeth. In most cases the length of time for such correction is the same whether braces or Invisalign aligners are used. Closing just the gap could take as little as six to eight months. But when there is a gap like the one you show, there are usually other associated bite problems that should be fixed at the same time. if so, the treatment could take longer. With an quick examination your orthodonitst should be able to give you a reasonably accurate estimate of how long your treatment will take.

With wires, it might be a bit faster, perhaps 6 months (if we are only talking about the gap). Invisalign will be around the same. At your age, comprehensive orthodontics would be best, as fine tuning the bite can be managed as well as facial development.
I would suggest a complete evaluation with a local dentist or orthodontist to find out a more clear estimate.