It depends -
The real answer is you bin the needle when you hear a thuck, thuck noise whether it's new or been sewing forever.
The received wisdom is about 8 hours of actual sewing time . . .
BUT
that varies depending on -
- the size of the needle (smaller sizes tend to dull quicker and can bend more easily than thicker bigger needles)
- the fabric (some synthetic fabrics dull the heck out of a needle);
or
- sometimes the thread being used (eg. bonded nylon, metallic, etc wearing a groove in the eye of the needle);
or
- the job your'e asking the needle to do eg. 8 hours at stitch length 2mm involves double the amount of needle penetrations of 8 hrs with stitch length 4mm Some synthetics can dull the heck out of a needle and so can stitching through lots of thick layers.
As mentioned above, the real answer is you bin the needle when you hear a thuck, thuck noise.
Often issues with needles become apparent because a needle of too small a size of needle is being used for the thread and type of fabric.
Also, once a needle has been used for metallic thread, don't use it for anything else.
Depending on what type of sewing you do most of the time and the fabrics you use, it makes sense to try and buy in as few different types of needle.
There's no point in scrimping on needles for the sake of a few pennies - often we've invested silly amounts of money in our machines and none of us want to ruin gorgeous fabric.
That said, there's a balance to be found between what's practical and works for you.
It helps getting to know when it's time to bin a needle.
Currently I stick to 4 types
Stretch needles (sizes 10, 12 mostly, 14) - for any fabric with lycra and all knits
Microtex (9, 10, 11 mostly) - for fine lightweight fabrics and some troublesome ones.
Titanium coated top stitch needles (Sizes 11/12 mostly, 14 & 16) - for all general sewing and machine embroidery and when using metallic thread (titanium coated needles last around 8 times longer than conventional needles). The truth is I use them for machine embroidery and now also use them for any general sewing that doesn't need a Stretch or Microtex.
ELX705 coverstitch & overlock needles (sizes 12 mostly & 14)- used for both overlocking and coverstitch for both wovens and knit fabrics. I can't be bothered with changing out different types of needles for O/L & cover stitch and keeping track of them. However, I do keep an eye on making sure the needles are the right sizes.