The ANP uncertainty (for want of a better term) grows mathematically not actually. It has been a long time since I did some work on this so my numbers could be out. On the B737 CL the loss of all GPS updating (our regulations require DME/DME and VOR/DME updating turn off for RNP AR approaches) causes the ANP to start expanding after 12 second coasting and then by .01 every 6 seconds.
Note that it is not a real ANP increase but mathematical "certification" ANP increase, the IRS system performs much better than that in reality.