There is a wealth of evidence supporting exercise as part of the treatment of depression. The NICE guidelines which all GPs should be up to date with supports this explicitly, and so anyone consulting a health professional with mood disturbance should expect to answer questions about how much exercise they do. It can be useful to quantify this, "exercise" is such a subjective term. Many people reply that they walk the dog each day, or walk to the shops etc. and want to know if this is enough. It is usually helpful to find out how strenous the walking is, if they are comfortably dawdling along then the social aspect, or just getting out of the house, may well be helpful, but as "exercise" not so much.
I knew that 30 minutes of relatively strenuous exercise in a day (ie enough to increase your breathing and heart rates) was found to be benificial for both mental and physical health. However, I only found out recently that it does not have to be all in one go, so people who have 3 lots of 10 minute bursts of exercise will benefit equally. This seems much more managable for those with increasingly busy lifestyles to acheive.

I hope this may be of some help.