Time limit is essential. I watched my sister-in-laws offer held for two weeks while the REA held two home opens.
All my dealings with agents suggests that as soon as they receive your offer they will immediately call other potential purchasers and give them an opportunity to put in an offer at the same time. The time limit lessens the agents ability to use your offer as a sales tool to extract a better offer from someone else.
Call you agent and give them a time limit and follow this up with an email. People can be contacted in most places around the world so there is no reason more than 24 hours would be required for a seller to accept, decline or counter your offer. If anyone needs more than a day to reply something is wrong.