Probably not, and it's a 250GB soft cap. If you repeatedly go over, they're going to start charging you for each additional GB, but you'll have to check with Comcast for the full details. Just be prepared to get the run around, since it took the FCC stepping in before Comcast would even give any specifics regarding this.
Anyway, the size really depends on a number of different factors. But I would go on the assumption of AT LEAST 175MB for a half hour show, 350MB for an hour long one, and 700MB for a movie. Odds are it will be higher than that, so you might want to try asking Netflix what sizes are typical.
Bit of an IMO rant to round this out... Comcast, and indeed every other ISP out there trying to impose bandwidth caps, needs to be smacked around and hard by the FCC. In their advertising, they often specifically mention the ability to watch high definition video content as one of the reasons to buy their service, but then if you actually do that, they penalize you for it. Not to mention the highly deceptive advertising practices, saying you can get speeds "up to" something or another, when in the fine print so small you almost need a telescope to read it, it basically says that you're never going to see anything even approaching the advertised speeds due to the realities of networking. All this because ISP's are too cheap to improve their infrastructure. Cable companies are the worst offenders. They have been happily overselling access to their networks from the beginning, and rather than actually increasing the network capacity, they do idiotic things like implement secret packet shaping programs, and bandwidth caps.
The FCC needs to step in and put a stop to these rampant consumer abuses. If a company wants to put a bandwidth cap on their service, fine, but they can't advertise it as "unlimited" any longer. They also have to make it very clear in promotional material what that cap is. I also think they should be required to create a new service tier that is truly unlimited. They can charge a higher price for it, that seems only fair, but these accounts are not subject to ANY kind of bandwidth caps or traffic shaping. I also think that if ISP's are going to implement bandwidth caps, they are obliged to provide their customers with a tool to monitor their usage. Clearly the ISP's have this ability, otherwise how do they know if you've gone over the cap? They should be required to make this information available to customers by phone, web, and even a downloadable program.
Anyway, rant aside... I'd contact Netflix, see if they can't provide you with some typical sizes for content so you can get a pretty good estimate. Just be sure you use lots of words like "typical", "average", and "estimate" otherwise they're going to try and feed you some line about how they can't tell you because every file is different. But if you make it pretty clear you're just looking to get a ballpark idea, and you're not going to hold them to a specific figure, they might be a bit more willing to help.
I've heard that comcast has a 150 gig limit on data every month. Is is safe to watch movies using xbox360+netflex and play heavy on line? I'm worried I might go over, does anyone know how much data is used to stream 30 mins episodes?

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