How long is the average short term inpatient drug rehab stay?

i was told my boyfriend would be in for 21 days but i didnt know if this was normal....
he hasnt been on the drugs for a long time its been abou 6 months i would say and he wasnt on any when he went in.

: http://alcoholdrugstreatment.info/how-long-is-the-average-short-term-inpatient-drug-rehab-stay