That all depends on the unit you are going to and their authorizations for E-5s. Usually you can expect to wait 30-90 days as something like an evaluation period for the unit to get a feel for you.
Since you are in the Reserve this could go on for 6 months to a year. However it is basically up to your chain of command. It is possible that if you meet all other requirements like: MOS qualification, Military Education(WLC, SLC, ALC,), Time in Service and Time in Grade that the unit will recommend you to appear before the promotion board in short order.