You don't necessarily go to jail for a long time for just possession, usually when that happens its after the 3 strikes law that some states have.
Federal sentencing guidelines take into account not only the amount of marijuana but also past convictions. Not all marijuana convictions require jail time under federal sentencing guidelines, but all are eligible for imprisonment. If convicted and sentenced to jail, a minimum of 85% of that sentence must be served. The higher the marijuana amount, the more likely one is to be sentenced to jail time, as opposed to probation or alternative sentencing. Low-level offenses, even with multiple prior convictions, may end up with probation for the entire sentence of one to 12 months, and no jail time required. Possession of over 1 kg of marijuana with no prior convictions carries a sentence of 6 to 12 months with a possibility of probation and alternative sentencing. Over 2.5 kg with no criminal record carries a sentence of at least 6 months in jail; with multiple prior convictions, a sentence might be up to two years to three years in jail with no chance for probation.