How do I structure node data that records time and quantity?
These are just my opinion...


A custom entity would probably be overkill, unless you plan to extend the functionality at some point. If your end game is just to store those key/values pairs, the extra code and DB complexity doesn't feel worth it.
To extend on that, Field Collection already implements that entity for you, plus all of the entity/field/views niceness. Add the Field Collection Table module as a table formatter, and you'll be probably be ready to go in about 10 minutes.
A custom field (and formatter for the table) would be reasonably straight forward to implement, but is tricky to extend if you decide, for example, you need another data column. It does have less DB overhead than the Field Collection method (as there isn't another entity involved), though, in case performance is a major issue.


If you're looking for simple, I'd say Field Collection is the way to go.