i dont think there is a definite time period--its a matter of convincing the case worker that your relationship is genuine and subsisting.

however--as the sponsor you have to meet specific requirements such as proof of earnings = £18600 p.a. with payslips over a set period--and bank statements for the last 6 months. is this likely to be a problem for you--? if so--you may well need to defer the application till these requirements are met.