I'm certain it varies by company and location, but most job offers I've received have had two dates:
- A joining date - the day they want you to start
- An answer date - they day they want an answer
In my experience, both are up for negotiation.
The goal of the company making the offer is to get an answer quickly enough that it will not cause chaos in their staffing plans. I've had everything from a "Can you tell us TOMORROW????" to "Take 2 weeks and let us know".
I'd say that if you don't get such a guideline, it's professional to ask when they'd like an answer by to clarify the situation. It can be a win or loose in either direction - the candidate can really mess up the company's staffing plans, or the company may move on when they don't hear a yes or no in a reasonable amount of time - so it's good to know what they think "reasonable" may be.
The impact on the company
Most times a company doesn't know or care what other offers you have in hand - they are pretty self-centered and just want an answer. Waiting a long time and then answering is annoying either way.
People say "no" all the time, and how you say "no" does matter. The more time and effort the company has put into hiring you will have a lot to do with how they take rejection. In particular, there's an intangible "pain in the rear-end" factor. For example, these to cases are very different:
You did the standard interview process, they flew you out, wined and dined you, and make an offer later. You said "no" pretty quickly - a day or two later - based on issues that were clear in the interview cycle. Outcome: mostly not a big deal, they knew the risks before they wined and dined you.
You did the standard interview process, it was a couple of drives into the local office, and meetings with a bunch of people in the team/department. They made an offer. You and the HR rep spent the next two weeks negotiating a wide and complex set of details about the offer, causing them to make numerous special exceptions to standard policy. Every time you communicated, you expressed your enthusiasm and excitement at the opportunity. Then you took a month to decide only telling them when you got a better offer. Outcome: You are probably on the "pain in the rear-end" list and they won't be quick to accept a resume from you.
I made these two examples to demonstrate a point - while sometimes companies will spend a serious amount of money recruiting for positions that can't be filled locally, they do that knowing the job pool (case 1), and they are taking a known risk. When you take massive personnel time from busy people (case 2) - you are are tapping a different resource, and one that will likely stick with the humans who had to deal with you. Particularly when it seemed like a go, and they had to do a lot of management decision making, you've upped the "cost" higher than simple dollars, and you'll be remembered unfavorably.
The impact on the candidate
And early acceptance is usually fairly low risk. Employment law varies - but a good faith offer is often something that holds up in court. Particularly a formal, written letter with a start date. There can be odd cases - like applications requiring heavy background checks, where a conditional offer is made - and usually your potential employers will advise you.
Certainly, it's no fun having an offer be retracted, but in my experience, it's unusual in a stable company.
I'd say a bigger risk is being spectacularly rude. People are social creatures, and they remember outstanding rudeness. I suspect that if you work in an industry with a very big pool of people that aren't tightly connected, the rudeness matters very little. But in a specialized skill, small working community getting a bad rep is a real minus. It'll follow you. For example as a digital security nerd-manager, I was really surprised at a cross-company, cross-industry local exec ed program to find that I had a 1-degree of separation (a friend/colleague of mine was a friend/colleague of the other attendee) with about half the people of a 20 person program. I had no idea my modest network extended so far, and was really, really grateful that ALL of those connections were friends, not enemies.