First Time

[timor]

With fixer its hard to over do it. I typically fix for 12 minutes, even if I might be able to get away with half that.

I am afraid that yes, one can over fix the film. Not so much in the matter of influencing the image. Bleaching may occur, but after really long time. Where too long fixing is not desired is the longevity of the negative. Longer fixing is leaving more of the fixer and fixing byproducts in emulsion requiring much longer washing times in water and in something designed to remove those byproducts, as they are not always soluble in just water. Those left in emulsion will destroy the neg in as little as a few weeks. Hipo or weak solution of sodium sulfite has to be used. Prolonged washing has an adverse effect on emulsion causing swelling which may rearrange grain with possible lose of sharpness. Can also cause much dreaded reticulation. (Happened to me with Tmax 400.)
Here is how to establish proper (minimal needed ) time of fixing (from post in APUG by MattKing):

"The "leader test" just means dropping a drop of fixer on an undeveloped film and leaving it there for 30 seconds. Then immerse the scrap of film in the fixer and time how long it takes before you can no longer tell where the drop was (i.e. the film is no longer milky and is fully cleared). If that time is reasonable (around one minute?), your total fixing time should be twice that for traditional films, and up to three times that for T-grain films."

With proper fixing washing time may be as short as 10 min. in slowly running water at the same temperature as the rest of the process. For me that usually 20C.