I've brewed many batches of beer all with the extract method. All Brewers Best kits. I've generally let them sit in primary for one week then secondary for another before kegging. I've always been a bit reluctant to check the gravity to see if fermentation has stopped by opening the bucket because I was told that oxidation can occur if you add too much oxygen to the buckets and opening it for three days or more in a row to check SG would definitely do that. I was also told that you should get the beer off the trub after a week as it could add off flavors. Lately I've seen that some leave it in primary for several weeks and it's not a bad thing... I'm confuded..lol. Please help
You don't have to worry too much about that, the answer is that you can, but don't have to. I have done both in the past.
Opening your fermenter to take a gravity reading is not risky if you sanitize your equipment properly.
There are reasons when racking to a seconday is beneficial. There is a lot of discussions about that topic here, please take a look at:
Most of us are not doing secondary. After many many batches in my life, I came to the conclusion that most beers I made were done after two weeks in primary. And by done I mean, ready to move to bottles or kegs. (The beers are actually done sooner than that with a good ferment, but 2 weeks fits my schedule most of the time and I KNOW its done at that point without worry... for most beers.)
I'd say you shouldn't fear oxidation from opening the bucket to get a quick sample with a thief or sanitized glass measuring cup. The amount of oxygen exposure racking to secondary is much greater than taking 2-3 samples for gravity checks. Eventually, you'll be confident/educated enough to not have to check so often.
Great question. Racking the beer too quickly can actually lead to worse off-flavors, because yeast can metabolize diacetyl and absorb sulfur, but when you rack then you remove like 95% of all the yeast. Concerns regarding yeast autolysis are overblown and antiquated. In reality, autolysis does not kick in for at least 6 or 7 weeks, and often much longer. As such, I recommend leaving the beer in primary until you're certain it's done fermenting, based on the krausen falling in and the beer beginning to clear... and then wait even longer, about 3 more days, just to be certain. From there, it should be safe to package straight away. No need to move to a secondary step -- just skip secondary. Millions of homebrewers have figured this out and are doing the same.
I've been brewing 6 years now and have about 120 batches under my belt. I have never moved a beer to secondary. I've listened to a ready a lot on this topic and general consensus is it is no longer necessary (if it ever was). I rarely let a beer ferment two weeks before I bottle/keg it.
As for gravity check - I used a long "straw" to take sample via the airlock port which limited O2 issues. For me the concern was about infection. Today I use a Tilt Bluetooth hydrometer. Not super accurate, but it will tell you when the fermentation is complete.
And to @dmtaylor's point, removing a beer from the trub/yeast before the diacetyl rest can cause off flavors.