Everybody knows the importance of immobilisation (plastering) of a fractured long bone. Using of retainer after fixed orthodontic treatment is as much as important. There will be remodeling of tooth supported alveolar bones during the tooth movement. At the time of removal of braces the tooth supporting bone will not be sufficiently calcified. It will take a minimum time of 6 months to 1 year. In some conditions it may take more than this duration (depend on age, pregnancy, tongue thrusting habit, mouth breathing habit, macroglossia etc..). The duration of retainer use is depend on somany other factors too. There are short period(6months full time after that 6 months night only), long period(2-3 years full time after that 3 years night only) and life time. This period can be decided by the orthodontist depend on the condition you have and he will inform you the duration at the time of debonding(removal) of braces.
Next Steps
In your case there is 2mm space. It is not possible to close the space using an active removable appliances without compromising the occlusion (accurate and stabilized relationship of upper and lower dental arches and tooth). It may leads to periodontal problems(gum disease and loosening of tooth) in future. So it's better to go for a fixed braces and use retainer as recommended by the orthodontist after the treatment.
Health Tips
Don't forget to use fluoridated mouth rinses during fixed or removable orthodontic treatment.
Do fillings of all developing Dental caries before starting of orthodontic treatment.
Do dental scaling once in every 3 months when you are under the orthodontic treatment.
Remove your all wisdom tooth before debonding if there is lack of space for them in the dental arches. it will also prevent late lower labial imbrication (a type of relapse).
thank you :)