The moisture absorbing capacity of a drier core is minuscule so changing them often will ensure your system is clean and dry, If it were my system I would install duel set so they can be changed online every 6 months or after any line opening/ compressor service, get in writing what vacuums were achieved after each service if you want to be finicky and scare your service company, this will ensure there are no quick purge short cuts, get oil samples taken regularly also, if this is done as well as other condition monitoring like VA you will save a fortune over the life of the unit ( no catastrophic failures ) this will give you excellent trending records and allow you to forsee problems and establish when issues had occured, So common with customers they will not do the $100-$500 repairs and it bites back hard however this is when the person in charge sacks the service company to cover his arse!