When cleaning your shower head of the calcium buildup by soaking the shower head in vinegar the minimum amount of time you need to soak your shower head in vinegar is 30 minutes.
But the longer you soak your shower head in vinegar the better off the shower head will be and it will clean more calcium out of the shower head.
Fill a bag with vinegar and then tie the bag around the shower head and allow the shower head to soak in the vinegar for at least 30 minutes.
However for best results you should allow the shower head to soak in the vinegar overnight and then in the morning turn on the hot water after removing the bag and flush it out.
The shower head should be clean and you should notice and increase in pressure from your shower.