How long can a person stay happy, excited and motivated about something new? I'm interested in learning more about studies or experiments that determine how long a typical person can stay excited about something new:

New job
new project
new relationship
new idea
new purchase/toy


Is there any evidence to support the hypothesis that humans are excited by new things/prospects, but only for a short time? If so, how short? Is it days, weeks or a "honey moon"? 

A while ago I've heard a talk on TED.com that happy people are up to 30% more productive (Does happiness affect productivity?), and I'm wondering if this is applicable to "being excited about something new" - i.e: are people more effective/smart when they are taking on new project, new job or a new relationship? Are people more excited, energized and engaged when they conceive new ideas? 

It would also help if I get to learn what the search term for "being excited about something new" is. 

Update:
Looking into happiness and productivity produced two terms: "Happiness economics" and Hedonic Adaptation. The article on hedonic adaptation says that happiness is homeostatic in nature 


  a system that regulates its internal environment and tends to maintain
  a stable, relatively constant condition of [its] properties


Reading that happiness regulates itself (as in negative feedback) sounds very reasonable to me, and I recall reading about antidepressants and Serotonin, which also mentioned that Serotonin production uses negative feedback, where more Serotonin released now inhibits release of more Serotonin, thus antidepressants dont work immediately and require a couple weeks to take effect[looking for a link]. 

I'm wondering if the process of being excited about something new is also homeostatic in nature?
Probably the most striking evidence of "happiness homeostasis" is a now classic study by Brickman, Coates and Bulman (1978) which compared the self-reported happiness of lottery winners and accident victims with a control group. The following quote describes the part of the outcome you'd be interested in succinctly:


  Lottery winners and controls were not significantly different in their ratings of how happy they were now, how happy they were before winning (or, for controls, how happy they were 6 months ago), and how happy they expected to be in a couple of years.


Except for one, the lottery winners were between one and 18 months removed from their winnings, while the group of accident victims were one to 12 months removed from their accident. This seems to indicate that yes, happiness is broadly speaking a homeostatic process (but as with most homeostatic processes, it's likely complex).

That's probably not the precision level you were looking for, but there's quite a literature--almost 1600 citations on Scholar for this paper alone. One other limitation of it is that it uses interviews instead of ecological momentary assessment, which a more modern paper would likely want to employ. That might be another term to keep in mind.

References


  Brickman, P., Coates, D., Janoff-Bulman, R. (1978). Lottery winners and accident victims: Is happiness relative?. Journal of personality and social psychology, 36(8), 917-927.