The frequency of peeling off the top layer of sticky mats is dependent on the amount of visible dirt and its loss of tackiness. The top sheet should be removed when it appears contaminated. The amount of traffic into and out of a cleanroom will determine how dirty it gets. Operators should monitor the amount of dirt collected and loss of tackiness and then write their own protocol to cover the intervals between removing the dirty layer and exposing a new clean layer. At a minimum it is recommended to expose a new sheet every shift. Some companies expose a new sheet with each shift entry (start of shift, after breaks, after lunch break, end of shift). Again, it is dependent on the amount of accumulated contamination and loss of tackiness.
The proper way to remove sheets is to slowly peel back the sheet starting in the corner where the tab is then lifting all edges and corners into the middle of the sheet. You want to fold into the middle to contain the contamination. Continue to fold into the middle until a new sheet is presented.
IEST-RP-CC18.3 Cleanroom Housekeeping: 5.2.6 is an excellent resource for sticky mats.