If you mean the current selling price of this stock is below the price at which you bought it, you have what we call an unrealized loss, not a capital loss carryover. You’ll report this loss on your tax return for the year when you sell the stock (assuming the stock price remains underwater). A capital loss carryover happens when stock or some other capital asset is sold at a loss that can’t be fully used that year. In either case, there is no time limit on using the loss, apart from having to be used within your lifetime.
There is one situation that is time-sensitive. If the stock has become worthless, you have to report it as being sold for $0 in the year it became worthless.