Although "maybe" looks like the words "may" and "be", it functions separately and differently, just like "however" is not "how ever".
It is also phonologically different. Maybe is /ˈmeɪˌbi/ with stress on the first syllable. May be is /ˈmeɪˈbiː/ with equal stress on both syllables. So we can clearly tell the difference between the two in spoken English.
"Maybe" is related to "may be" the sense that the pattern:
can usually be rewritten, and can perhaps be regarded as a contraction of:
It may be that [clause]
The "it" is a place-holder subject, just like in "it is raining" or "it is said that ...".
"Maybe" is a fairly flexible particle which applies to entire clauses ("Maybe I should wait"); verb phrases ("I think he maybe went home already"), adverb phrases ("I can finish that in three hours, or maybe sooner"); adjectives ("I don't remember what color it was: maybe red, maybe orange"); or nouns ("I see a vehicle in the distance, maybe a bus, coming this way"). "May be" cannot be in the same way, simply by placing it in front of a clause or phrase. It is a compound verb which requires the formation of a clause with a subject. If we replace all these uses with "may be" we have to rearrange or complicate the sentence so that there is a proper embedded clause built around "may" with a subject: "It may be that I should wait"; "It may be that he went home already"; "I can finish that in about three hours, or it may be that I can finish it sooner"; "It may be that it was red; it may be that it was orange"; "I see a vehicle, which may be a bus, coming this way".
Maybe it's time to switch doctors.
can also be expressed as
It may be that it is time to switch doctors.
possibly even with the complementizer omitted:
It may be it is time to switch doctors.
But "May be (/ˈmeɪˈbiː/) /it's time to switch" is not grammatical. The verb "may" is lacking a subject.