To answer your question about how many generations is safe to use the same bull....
The natural percentage of blood from each parent in the resulting offspring is 50%. You for example, carry half the genetic influence from your father and half the genetic influence from your mother. In other words you have 50%of your father's blood (don't know why they use the term blood, but it is widely used that way). When it comes to breeding cattle it is generally accepted that if you do not exceed this 50% guideline you should be OK.
Just bear in mind that the whole idea of linebreeding is to create homoygous strains of cattle, in other words through selective linebreeding and VERY STRICT selection. Culling everything that shows any flaws doesn't matter how minor. This way all the faults that might be hidden because they were carried by a recessive gene will be exposed in subsequent generations. If you culling stategy was in place you will end up (hopefully) with a pure strain homozygous cattle without any hidden flaws that might be hided by recessive genes. Of course you can use the same method to fix any other wanted trait in your herd as well.
The flaws that are likely to show up through intensive linebreeding are overshot/undershot jaws, sickle hocks, straight hocks, etc. Just your normal cull faults. You definatly will not get two headed calves or calves with eight eyes or whatever horror stories you might have heard.
what I forgot to mention was you need to linebreed to a superior animal (usually a bull) that already possesses the qualities you're after. You can concentrate gene pairs this way, but you can not create new ones.
Jim Lents wrote a book called "The Basis of LInebreeding" that can shed light on any otehr questions you might have in applying this technique in practice.
Hope this rather lengthy explanation helped at all, if you would like me to clear anything else up please ask.