Firstly, it should be noted that the effects of HFD depend upon the mouse strain selected (some are resistant). Secondly, HFD (60% fat) impairs glucose-stimulated insulin release even after one week of feeding (as determined following IPGTT/IVGTT) (http://diabetes.diabetesjournals.org/content/53/suppl_3/S215.full.pdf), although most people will investigate this at 6-8 wks. Both fasted insulin and glucose levels should be elevated, the likely explanation being the onset of compensatory beta cell expansion in the face of insulin resistance (the beta cells however don't respond normally to increased glucose; see above). Severity of the latter can depend on genetic background and can be crudely tested using ITT (or taking liver tissue and checking for insulin signaling). Beta cell failure occurs from around 18 wks of HFD onwards, again depending on genetic background. It is worth bearing in mind that effects of HFD depend on source of animals, gender, age, housing environment and diet manufacturer, so it is important to define the norms in your lab.
What is the model you are using? As David mentions, strain selection is very important. Some models will develop hyperglycemia and hyperinsulinemia very quickly upon HFD feeding, but eventually lose the hyperinsulinemia. I worked with a model that is hyperinsulinemic even before HFD, then both insulin and glucose would peak within a week or two of HFD feeding, but then insulin would drop off and be nearly undetectable by the 6th week of HFD feeding, though as you can imagine, BG remained quite high.
High fat diet will lead to higher levels of circulating free fatty acids, which can affect insulin release as well as insulin resistance. Please see publication by Shang et al (2002) Pancreas 25, 393-399. They had als shown that insulin secretion is enhanced during glucsetolerance test in Wistar rats fed high fat diet.
Your question: High fat diet affect pancreatic insulin production levels, kindly go through the following link: I hope these links are very helpful to you. I think so.
There are differential effects depending on the length of time of diet. In response to a high fat diet, there is an initial hyperinsulinemic compensation - the pancreas releases more insulin, and the liver also reduces insulin clearance, in an effort to correct insulin resistance. There is a time course in dogs shown in Mittelmann et al (2000) Diabetes, and these results are evident by 3 weeks (IVGTT). Similar to what Elizabeth and David said above, it would really depend on the model/strain, but also the length of time on diet.
As David wrote, after a long time of high insulin production, the beta cell fatigues and can no longer produce insulin, which leads to lower levels of insulin, though in humans this is typically after a long time with diabetes (patients typically begin using insulin). As Chandra and Senthilvel say too, the trigger for this may indeed be fatty acids, and we have preliminary data showing that different types of fat (SFA vs omega-3) may have different effects on insulin release by the pancreas.
In short, the type of diet, the length of time, and the model/strain may all affect your results, and insulin levels can reflect both production and clearance.