You have to judge based on the urgency of needing the reply, how urgent the item might be to the person who needs to respond, and whether nudging them will be more detrimental rather than less.
For instance in the job appliaction, nudging them will not make them move faster and is likely to make them mad at you and reduce your chances of getting the job if you are a pain about it.
But suppose you have a work issue where something is failing and you need a response in order to fix it (for instance I can't work on failing feeds unless I have a job to charge the work to). If I have people who can;t log in becasue the import has failed and teh new users have not been imported, that is critical, so I would follow up within a couple of hours if I got no response to my email request for a job ticket to work on (Probably by phone this time). If I was working on an RFP (Request for Proposal) to compete for new work and needed some information from somone, I would try to ask for it in time so that they would have at least a week to respond but the would followup at least a couple of days before the immoveable deadline of an RFP.
If I was working on something that I needed a response to and was not getting one even after requesting more than once, I woudl escalte to the next higher level. YOu have to remeber in business people are not sitting around waiting for you to send them requests and their priorities may not align with yours. When someone failes to respond, escalating will ensure the priorites are realigned or you will be told not to bother this person because his priorities are genuinely higher than yours. Generally you should only escalate if the task is genuninely important or if the person is rountinely non-responsive to your requests.