When you complete an offer/survey, the advertiser's system pays FC. When FC gets paid (notice of a credit from the advertiser), you get your credit. This is mostly done automatically with the computer systems in place and the checks and balances that the advertiser has to confirm that you should get credit.
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If that communication breaks down, then chances are neithor FC nor you will get credit. Which is why it's important to leave confirmation windows open for some time to allow the information to flow smoothly.
It's also important that you clear your cookies and temp files BEFORE you begin a new offer.
Some offers will be paid quickly, but other offers may require a longer time frame because the advertiser wants to make sure that you follow through correctly on the specs of the offer.