It's not like a table wine, which can taste pretty rough if it's been standing around for a week. When you open a sherry, it will start (very, very slowly) to lose its taste and nose (smell). But ... you can keep going back to it for several weeks, and not notice the difference.
After a few months, it still won't do you any harm, it just won't be as exciting as when you first opened it. A sweet sherry will be less rich and mellow, a dry sherry will be less crisp and fragrant. A dry Fino will have the advantage of being kept chilled in the fridge.
You can meet some (frankly) terrible wine snobs who will have a checklist of "rules" but, basically, if it still tastes okay, drink it. It won't kill you. And you'd have to develop a very discerning taste for sherry before you could tell the difference. And to develop such a taste, you'd have to drink it regularly, so it would never get left for very long in the first place. That sounds like a "Tah Daah" moment of logic.
Anyway, if Granny wanted a sweet sherry at Christmas, the half drunk bottle from the previous Christmas would frequently come out from the back of the cupboard. It never did Granny any harm. Mind you, she is dead.