How much data does a session/data key typically encrypt? I believe the masker key, $MK$, of a block cipher might be used for several years and is used only to encrypt $\textit{key encrypting keys}$, which in turn are used only to encrypt $\textit{data keys}$. These data keys are the ones actually used in cryptographic computations and are often also called $\textit{session keys}$. They have short lifetimes and may be used only for a single session. My questions are:


What is a 'single session'? 
How much data might the same session key be used for (max or min?)
How long would a session key be used for?


I know answers to these questions depend on many things, such what is being encrypted, how long must it remain secure, etc. but just some ideas or real-world examples would be helful.
The most common encryption protocol is probably TLS, as used in HTTPS. AES (with a 128-bit block size) is probably the most common encryption algorithm. Therefore the maximum amount of data that can be sent safely is $2^{128/2}=2^{64} blocks = 256EiB$. 

Typical sessions end when the user closes the web page, and so send significantly less data. Even binge-watching an entire season of a TV show on a streaming site will only use a few GiB for a session. 

Of course some users rarely close their browser tabs, but the most common PC OS is Windows which updates requiring a reboot at least once a month. Thus the longest duration of a session is likely to be around a month. 

This answer has ignored session resumption, because it isn't that widely used. Even where it is used most protocols (including TLS) provide a way to re-negotiate keys. 

TLS 1.2 recommends that sessions last no longer than 24 hours (section F.1.4).