How long does bell's palsy usually last?

Days to months. Recovery from bell's palsy is quite variable. It can take from a few weeks to many months to recover facial movement. Fortunately, most people will regain normal motion eventually.
2-5 wks+/- Most bell palsy cases will fade within the first 3-4 wks although a percentage (3/20) can have some mild residual weakness &(1/20) may have residual severe facial weakness. I send most of my cases for ENT consult eval early as some rare problems may have similar facial droop.