COMPARISON OF ADJECTIVES
Comparisons of Quantity
To show no difference: as much as , as many as, as few as, as little as
as many as / as few as + countable nouns
as much as / as little as + uncountable nouns
This is most useful, but I am not so sure it applies directly to the sentence at hand. Note that the various examples given on the site you are referring to (
English Grammar: ADJECTIVES : COMPARISON OF QUANTITY - AS MUCH AS, AS MANY AS, AS FEW AS, AS LITTLE AS)
are all of this type, "They have
as many children
as us."
Deepurple has brought something different however:
"The shortfall is so severe that the wait can be as much as four years."
Now, if this sentence were, "The shortfall is so severe that the wait can be as many years as four," the rule would most certainly apply.
However, in the sentence at hand "as much as" seems to refer to "the wait." In that sense, I believe that either form would work. We might consider the sentence to be short for, "The shortfall is so severe that the wait can be as much
time as four years."
I would therefore accept either "as much as" or "as many as."
I hope others will join in on this little controversy. I confess that I feel I am standing on less than firm ground here.