What if the Polish-Lithuanian Union and Muscovy unite in a personal union with Fidor I accepting Polish demands and therefore forming the Triple Union/Polish-Lithuanian-Russian Commonwealth? How long could it survive? Just how powerful would such a state be?
As in Fyodor I? If it's before the birth of his daughter and that daughter is instead a son, then it could last. However, Russia would be a nation that was recovering from the lengthy Livonian War, Oprichnina devastation, and the Crimean sack of Moscow. Also, the Rurikid succession would be in a similar fashion to the Jagiellonian one, if Fyodor dies without a male heir as in OTL then the whole union is thrown into chaos.
All 3 nations had a strong emphasis on local nobles. In Russia, the boyars were recently returning to an easier existence where they were not forced into two opposing factions, the boyars that comprised parts of the Oprichniki and were favored by Tsar Ivan the Terrible, Fyodor's father and predecessor, and those boyars who were apart of the Zemksi Sabor or assembly of the land. Getting a stable enough succession would be key, as for power the sky seems to be the limit, that it could give the Ottomans a run for their money.
The first challenge it would face would be Sweden and Crimea, would would certainly not tolerate this happening. As long as it can stop that, there's nothing that can touch it until the industrial era. The main difference that, as the ruler of Poland, such a country would become involved in western European affairs much sooner and more closely.
Crimea was rather weak, they could conduct raids and could be beaten by a significant force, like with what happened to Crimea after they managed to sack Moscow in 1571, it would be the Ottomans that would be a problem. The biggest problem would be getting such a state to work, you would have the questions of local nobles, religion, and size to deal with. Russia extended to control most of the Volga River trade with its conquests of Kazan and Astrakhan, it also gave Russia plenty Muslims and ethnic Tatars in their lands.