When/How often should a cot mattress be replaced in an Early childhood centre? 5 or 10 years? Or does it not matter as long as it is in excellent condition?

Red Nose has no time frame for replacing any cot mattress…...it depends on the type of mattress & how often it is used.
As long as the mattress remains firm & clean, has no signs of wear & tear or stains or mould then there is no reason why it cannot still be used.
The base of cot or mattress should not sag as this may form a trough particularly with a soft foam mattress into which a baby on its tummy may roll.

have a look at the information provided on Red Nose website (link below) for research behind recommendations for mattresses.
https://rednose.org.au/article/what-is-a-safe-mattress
https://rednose.org.au/article/second-hand-mattresses

Check with manufacturers instructions on each specific mattress.