Has she actually asked you to do 5 full days? I don't think she's expecting it probably just assuming you would help as her mother? im obviously assuming dad isn't on the scene/doesn't help?
I think YANBU to reject 5 full days- you are not a nursery, however I will go against the grain and say many kind gp's look after 2/3/4 GC on a daily basis and cope because they want to. unless you had your DD later in life I assume you aren't 'old' at all, i'm similar age as your DD and my mum is mid fifties and is as fit as me?
2 yo are hard but not impossible!
however, you ABU to come online and basically slate your daughter for wanting some help off you when she is literally doing everything she can to provide a better life in the long run for her child, something that should be a priority to you as well, as a mother and grandmother, surely you would want that?
and yes, he will be in pre school and then school when shes in university, whether that's in 12 months/18months/2 years, any uni course is usually 3 years, so it would be temporary.
maybe talk and compromise to your DD before coming to strangers when I think you've already made your decision.. if you cant wont help just say no? but then don't be surprised if she comes to you asking for help financially for childcare or because she's had to sack uni off due to lack of childcare..im sure at some point when your DD was young you struggled with childcare, unless you were a SAHM.
its hilarious seeing people say 'its her child she should look after it' on threads, like once you have a child you're completely on your own. maybe parents should be there to help and support when they can, not allow their own children to struggle..