Hi Yongkun, a hairline fracture should heal reliably given 1-3 months, but it may take extra time. The fracture might appear to be completely healed, but keep in mind that the bone needs to remodel 1% per year. Every day you wait will give you one more week until the bone is back to 100%. If you start "bearing weight" early enough after injury, then bolster with immobilization once extra stability is needed for support it should be fine.
Of course there are other factors that can influence how well a bone heals like smoking and diabetes which can increase your risk of added complications - so avoid these behaviors if possible by quitting smoking and taking care of your blood sugar before a break or fracture occurs!