Oxygen level drops below 90% stays in the 80s% for about 5 minutes then goes up to normal. what's the safest amount of time for oxygen to stay in 80s?

No such thing, O2 sats under 90 are usually supplemented with oxygen while workup for cause and possible treatment takes place, the silent killer is CO2 that usually builds up and results in narcosis and that is not commonly measured.

Your pulse oximeter may be inaccurate or your arterioles in the finger may be cold or vasoconstricted. If you're truly concerned, have pulmonary function testing which will include an accurate oxygen saturation result.