A USPS First Class package takes between 3 to 7 days to reach it's destination once you send it off.
The actual time it takes to get the USPS First Class package depends on how far away it has to come from and how busy the sorting facilities are and weather conditions.
Although USPS will make a very good attempt at delivery through rain, sleet, snow, hail etc it does sometimes get so bad that the USPS cannot send their trucks out on the road to get packages to their destination on time.
It's rare this happens but sometimes bad weather can make packages take a few days longer to get to you than it normally would.
Most USPS First Class packages I've had sent to me got to me within 3 to 4 days at most.
But in rare cases due to weather related events I had a few USPS First Class packages and other USPS mail take up to 7 days to reach me.
If you didn't get the USPS First Class package within 7 days then you can contact the shipper of the package.