Like I said, it's nearly impossible to assimilate for the person who was raised in another country. Far too many things are ingrained at early age; the best we can do is consciously mimic the norms of the new society, but that kind of self-control tends to be blown away in stressful situations.
That is not to say that one doesn't acquire a lot of new habits. Not all things are that deeply ingrained, and even old dog can learn new tricks. The result is that most people who have lived for several decades in another country still don't look like natives there, but they also become different from the norm in their country of origin, sufficiently so that it is immediately evident from their behavior if they ever visit that country.
It's especially funny when it's a person from the "returnee" category who is disparaging the culture from which they've returned - there was one coworker of mine like that, who returned from the States after living there for 17 years. Every time he went on another rant about how stupid and soulless Americans are, speaking Russian with a definite American accent (occasionally slipping into English when he couldn't remember the Russian word for what he wanted to say), and accompanying it with distinctly American hand gestures, I couldn't help but chuckle.
The real criteria for assimilation is the second generation, children who were either born in the new country, or arrived there as kids. In families who want to assimilate, such children usually do so, and, short of appearance, you'd be hard pressed to tell any difference between them and the natives. This happens naturally even if their parents are mostly ambivalent about the whole thing.
In families that consciously resist integration, the parents actively preclude their children from assimilating by restricting their social circle to children of other similarly-minded immigrants, speaking to them only in their native language (and reprimanding them if they speak local language at home or in other social settings restricted to their cultural group), trying to limit their access to local mass media and products of local culture (TV shows, music, movies etc), and so on.