I received an offer from a company and I need to respond back regarding my decision. However, I'm still waiting to hear back for 3-4 orher companies regarding my applications. I was wondering how late I can delay my decision? Does 2 weeks seem reasonable?
Rather than ask for a deadline, it is better to state the time you will need to make a decision, or even better than that simply state that you will be unable to make a decision until you have received and compared all of the offers you are currently expecting.
The reason is that if you ask them to set a deadline, you invite them to state one which does not work for you.
While if you state your own deadline in advance, you may find yourself unable to meet it.
There is of course some risk in any course of action, but it is likely that if you are a strong candidate for the first position they will exercise some patience - or at least if they cannot, then they will reply with a specific requirement and its explanation (ie, that they actually have a second choice they will offer the role to).
How late you can actually delay depends a lot on information that you have not stated and may not be known. For example, if they are hiring multiple people into a pool of similar staff positions from which project assignments will then be made, there's likely a lot of flexibility. Similarly, if you are uniquely suited to their needs, they will probably work with you. On the other hand, if they need one relatively interchangeable person for a specific task in the near term, then they'll probably want to soon move on to someone else who can quickly give them an affirmative answer.
My advice is to just ask for a deadline and, depending on what you think about the professionalism of the company, be open about waiting for other company's responses.
E.g. for my current job, I received the job offer via email with a disclaimer that it is only valid for three days. I totally forgot about it until, after a month or so, I was approached by the company saying that they needed an answer NOW. I responded with asking for another week because I hadn't made up my mind yet, and then accepted the offer on the last possible date. This was for their graduate program so they in no way needed a new hire immediately.
No one can say. Depends on the situation (urgency) for the company, how valuable your skills are and the type of position.
E.g. if you are a very programmer who will work in a big team, the chance to delay it may be larger than if you are a highly specialized person who would start to work alone on a specific project.