How long, is too long, on the market?

[Marty Gold]

I've heard people say a listing is old or stale or that's it's been on the market too long.  I'd assume those are house's which likely get reduced in price or possibly would be willing to negotiate a bit more liberally.  But how long is too long?  I'm sure all markets are different and year to year may change.  But is there a generally accepted rule of thumb anyone can offer?

Ty