There is a domain that I would like to buy that has a status as follows: Registry Status: redemptionPeriod
I presume this means the current owner has a grace period during which they can repurchase the domain but it is not available on the open market.
How long is this period?
It depends who the domain is registered with. ICANN who oversee domain registration say
3.7.5 Registrar shall register Registered Names to Registered Name Holders only for fixed periods. http://www.icann.org/en/registrars/ra-agreement-17may01.htm#3
but don't specify the period.
Nominet, who oversee most .uk domains, allow 83 days which includes repeated reminders.
With Network Solutions:
Typically, we provide a 35-day grace period during which the current holder can still renew it for the standard renewal fee.
http://www.networksolutions.com/domain-name-registration/pending.jsp
See who the domain is registered with (various "whois" services are available) and then see what the registrar's policy is.
Here is the of generic Top Level Domain life cycle from ICANN: 
So, it is typically 30 + 5 days. However as they state:
Some registrar activity post-expiration may not be reflected in the life cycle chart above.
Thus, I guess, it's not strictly defined.
