How much of my ancestry comes from each grandparent?
On average, 25%, because you have 4 grandparents, and all of your genetic material came from them as a collective. Of course it's possible to be more genetically similar to a particular grandparent by random assortment of chromosomes, but the average will be 25%.

Genetic tests can't tell you what your ancestry is (companies implying this are probably misleading you on purpose). At best, they can tell you how correlated certain features of your genome are with people from a certain geography. Even people who are currently live in Eastern Europe will not test as "100% Eastern European." There is far too much mobility and breeding of populations across geography and far too much variation within populations.