When we put together an offer on a property most buyers ask how long the seller has to respond... and are surprised there is no time limit unless it is specifically written into the offer. Most sellers respond within 24 hours or sooner, but that isn't always the case.
So... does it make sense to include a deadline for the seller's response? Sometimes it can make sense... for example, if you need to find a new home quickly and have backup properties in case your first choice doesn't work out. You might also impose a deadline if you make a strong offer shortly before an open house and you fear the open house might generate competition... but that can also backfire in these days of multiple offers. What if they decide to wait, are you prepared to withdraw your offer?
In most cases I don't recommend setting a deadline. You never know what the seller circumstances are and it may be difficult or impossible for the seller to respond within your imposed timeframe. If so, then what do you do? If you aren't prepared to withdraw your offer it doesn't make sense to set the deadline in the first place because it will be seen as an empty threat and weaken your negotiating position.
Also, in these days of many multiple offers sellers often choose to wait a couple days to give multiple buyers a chance to write offers, then review them all later... especially over the weekend. This is following the trend of bank foreclosures... bank owned properties often collect offers during the first week it is on the market to review at the end of that week.