Opening primary fermenter

I am on day 7 of my fermentation, and I'm wanting to start taking S.G. readings to see when I should move to a secondary. I'm wondering though, when should I start checking the S.G. and how often?
It seems to me that everytime I open the primary fermentor to check the S.G. I'm loosing the C02 buffer and letting a new batch of oxigen in contact with the cider. So I would assume that the less I open it the better. So I wonder when I should check for the first time, and how often there after?
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Primary: Apple Cider
Secondary: Empty (the cider is my first batch ever)
Bottled: Nothing (I told you I'm new)
Drinking: Budweiser (I told you I'm just getting started, why can't you just leave me alone)

Start at one week and then every three days. If you swirl the fermenter after taking the sample, CO2 will come out of solution and re-fill the head-space. It's heavier than air, so the air gets pushed out.

david_42 said:

Start at one week and then every three days. If you swirl the fermenter after taking the sample, CO2 will come out of solution and re-fill the head-space. It's heavier than air, so the air gets pushed out.

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Thanks, I figured 7-10 days for the first time, but wasn't sure how often after that. I'll use your sugestion.
__________________________
Primary: Apple Cider
Secondary: Empty (the cider is my first batch ever)
Bottled: Nothing (I told you I'm new)
Drinking: Budweiser (I told you I'm just getting started, why can't you just leave me alone)