For those folks lucky enough to afford the luxury of a cleaning service, tipping is generally viewed as an expected gratuity. It really comes down to the relationship you’ve created and how frequently they clean for you.
My experience is that if the cleaners visit weekly or bi-weekly, the cost of one visit is generally considered a good tip. You can increase the amount if that person works more often or even put together a gift basket of goodies during the holidays if there is a team.
You should consider throwing a larger tip if you have them doing any extra services that aren’t part of the standard package. Having them help with the laundry and ironing or sweeping your porches and driveway would qualify as above and beyond the normal duties.