if l dont wear the monthly lens everyday should l still replace them after 30 days?

yes because they have an expiry date once opened a cheaper option l found is the daily dispsables l get 30 pairs deliverd every 3 months for £9 as l only wear them once or twice a week.

No, your monthly lenses are good for about 30 days wear within a 60 day period, and still be safe. After being open for 2 months, they probably wo not be comfortable anymore anyway.

There is no 30 day expiry date that makes them no more good suddenly.