Dx'ed last year b12 deficient, level 169, receiving monthly shots now. could this cause me to have a cold longer than usual, it's been over 2wks?

Possibly; take more : Quoting ronald hoffman md "b12 is also central to immune processes because it governs cell division and growth. Without adequate b12, white blood cells can't mature and multiply." (http://www.Drhoffman.Com/page.Cfm/125) my favorite remedy for colds is pelargonium sidoides, widely sold as umcka. Monthly B12 shots maintain a normal level but i advise doing them 2-3x/wk til your level reaches 600!

No: This will not make your cold last longer. See your physician for recommendations and evaluation.