Depends. Thyroiditis is generally categorized into painful and painless type. In the former, occurring commonly during an upper resporatory viral infection, your thyroid levels may go high followed by resolution and normalization of thyroid levels in 4-6 weeks. In the latter, you have a greater risk of developing hypothyroidism ~ 5-10% per year of developing under active thyroid (i.e. Hypothyroidism).
Depends on cause. It depends on the cause of thyroiditis. Subacute thyroiditis causes intense neck pain but usually resolves in 2-5 months with no hypothyroidism. Hashimotos thyroiditis involves little neck pain, maybe a goiter, and usually makes you hypothyroid, although your thyroid levels can go up and down. Post partum thyroiditis is also variable.
Average 3-6 months . The thyrotoxic phase lasts 2-3 weeks, followed by a gradual decline in thyroid hormone levels. A hypothyroid phase follows and lasts a few more weeks, then levels return to normal.