I just called a few of my credit card companies (not the m/c or visa's, just store cards) and requested credit line increases. All but one was granted (Best Buy). Sams Club & Mervyns I was able to request online and it was immediately updated. Lowes & Circuit City I had to call an automated line and was told immediately that it was granted and it was updated immediately. Does anybody do this on a regular basis? If so, how often can you make these requests? I would like to start doing them on a regular basis.
It differs with various credit issuers but it seems like the normal interim is six months. If you use the card regularly this can help as well, assuming you pay on time, lol. Good luck!
