For regular donors, 56 days is the limit after which they can start donating blood again. As such, most people can replenish blood within 28 days to 56 days, so that is the prescribed number of days that one should wait before they could donate blood again.  On the other hand, plasma can be donated after every 28 days only. A person can donate plasma as many as 13 times a year, whereas for donating  platelets you need a gap of 7 days, and you can donate plasma for over 24 times in a year.  The same is 112 days gap in case of red blood cells, which can be donated 3 in a year.