Depending on the temperature of the liquid CO2 in the cylinder, it will read around 400 PSIG or so. The pointer will not drop until all of the liquid is gone and then you do not have much gas left. This is why keeping the cylinder outside of the refrigerator is advantageous since it is easier to determine if you are low on gas. At ambient, a correctly filled cylinder should read 825 PSIG and again, the pointer will not drop until the liquid CO2 is depleted.
The change in pressure at the keg should not take more than a minute. As to foam, that depends on the past condition of the beer and the state your system is in. If this is a fresh keg that has acclimated to your kegerator, the foam issue should subside as soon as the keg pressurizes at the higher level.
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