Unless you are very careful and use a stylus cleaning brush and cleaning fluid (usually some kind of pure alcohol) you WILL get build up on your needle. This affects quality.
Over time the needle point will wear down. This also affects quality/tracking.
Super expensive audiophile needles have diamond coated tips and this prevents wear, but aren't good for DJ'ing.
There are, however, heaps of other factors that affect performance of your needle. Such as whether you have a curved or straight tonearm, how many grams of tracking force you have wound on the tonearm weight, whether you carefully place the needle on the record or just dump it.
Traditionally curved tonearms are used for better quality sound but provide more sideways tracking force, whereas straight tonearms are designed for DJ'ing so as not to produce nasty sideways forces that cause the needle to jump.
So there is no definitive answer to your question other than - if you have been taking care of your needles they should still be fine. If your the heavy-handed type, or your cartridges are misaligned or you lent your dex to your m8 so he could learn to mix...... then it might be time to put new needles in the shells.
