That depends on the wallet you are using. Most likely you must do something actively to "claim" your new altcoins - the most common way is installing a "forkcoin client" and import the private key there. But you can do that whenever you want, there is no deadline (so there is no need to hurry).

Only if you are using a centrally controlled online wallet like Coinbase and the company has explicitly stated that it will support both coins and credit them automatically, then you will receive the coins without having to do nothing. That happened at some exchanges (e.g. Bittrex) when Bitcoin Cash forked.

(But you really shouldn't use such services. Electrum, for example, is almost as easy to use as blockchain.info.)