Hi,
There is no industry standard for how long after you assume a mortgage you would be able to refinance. It will vary from lender to lender. But in most cases, an assumption is not treated like a presale. You will have paid relatively close to market value, plus or minus the value of the existing mortgage. There would be a penalty in most cases, and you would have to qualify under normal bank guidelines. From experience, it is easier to qualify for a purchase versus a refinance, so you would want to weigh your options closely.
Similarly, there is no rule to how long you must live in a property, approved under a high ratio program, before you can rent it out. But the mortgage insurers will not allow this to happen more than once or twice. I have had clients that bought a property and then moved to another property a couple of months later, and there was no problem. He tried to do it a 3rd time, and was turned down by both insurers.
Hope that helps.