I'm coming for 5 or 6 months in the winter and have been looking at getting a long stay visa from the embassy here in Canada, but now am wondering if there is any reason I couldn't come in on a 30 day stamp, extend for 30 days, and then either do a bus border bounce, or grab a cheap flight to Cambodia or Vietnam for a weekend, and then fly back in, get a fresh 30 day stamp, another extension, etc.? Is there a limit how many times they'll let you do this, or could they just decide I'm abusing the system and deny me entry?