How can I extract the pictures from the camera memory of my Canon 550D?

SOURCE: Why does the canon 550d stop video recording

There are two possible reasons. The first one is a file size limitation. The size of a file is stored as a 32-bit number, so when you get to 4Gb, the file size is 'full' and can't be a bigger number; thus, if you're recording in HD you might get to this 4Gb 'brick wall' first. I think it comes at about 12 minutes.

The second is an artificial limitation made for tax purposes. Even if you choose a lower resolution, your recording will stop at 29:59, or one second short of 30 minutes. This is because the EU imposes a hefty tax on any device they call a 'camcorder'. They define camcorder as something that can record for 30 minutes or more. Therefore they stop the recording at 29:59 to avoid being defined as a camcorder, and hence avoiding the tax.

SOURCE: When i am taking picture

This is normal behavior for taking a picture using the bulb. After you release the bulb, the camera may detect that a noise reduction step is necessary. The noise reduction process can take as long as the exposure time. There is an option to disable long exposure noise reduction. Click here to see the manual for details.

SOURCE: I have heard that Digital cameras have a limited

Any mechanical device will eventually wear out, and a DSLR has many moving parts. The shutter and the mirror mechanism are prime examples, but in addition there are buttons and dials.

There's nothing in the camera keeping track of the date or number of pictures and then stopping operation when some limit is reached. In normal operation, the camera should be good for years and/or tens (or hundreds) of thousands of pictures.