There is a field "call notice days" or something similar on Bloomberg and similar bond indicative data databases. If this field is not populated, then assume 30 days. The bond issuer must give at least this many calendar days' notice if they choose to exercise.
To find the yield to worst of an American or Bermudan callable bond, the common practice is to loop on all potential call dates from today + call days until maturity, calculate the yield if the call is exercised on this date, and find the worst yield possible.
Hence, likewise, the earliest possible American exercise date is today + call days, so you use that date to calculate yield to call. For a bermudan, use the next date that is >= today + call days. This is consistent with what Bloomberg terminal does.
The issuer can't announce a call sooner that call days.