With food, I'm not sure if I should use 'is' or 'are' ?

1. Two slices of pizza is enough for me.

2. Two slices of pizza are enough for me.

3. Ten hours is very long. (I get why I use 'is' here. It is because 'ten hours' is one whole thing. What about pizza slices? Can I do that too or no?)

For me, the strength of the plurality of two slices requires the plural verb are. Slices aren't exactly units of measure, although someday the pizza industry may standardize the size of a slice.

For amounts considered as a whole, I referred to Mary Ansell's English Grammar: Explanations and Exercises (available on-line):

b. Amounts considered as a whole
A noun subject naming a unit of currency or a unit of measurement takes a singular verb when the amount referred to is being considered as a whole.
e.g. Ten dollars is my best offer.
Five minutes is all that is required.
Two years is a long time.
In each of the above examples, the amount of money or length of time referred to by the subject is being considered as a whole. Thus, in each case a singular verb, is, is used.

So I agree with your Ten hours is very long.
Personally, I prefer "two slices of pizza is enough for me," because your are talking about the combined amount, not the individual slices. If you were talking about the individual slices you would use a plural verb: "Two slices of pizza are bigger than the others." (Each of the two slices is bigger than the others; however, you could not say that each of the two slices is enough for me, they are only 'enough" when considered together.)
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I see your point, khoff, but I still lean toward are with slices, by analogy with helpings: two helpings [or servings, or portions] are enough for me. I think it's a close call and hope someone can point to an authoritative reference for the current standard usage.

One of my grammar books is saying that when a plural noun implies 'a whole amount', it is taken as a single entity and therefore followed by a singular verb.
Duration: Three weeks is a long time to walk for an answer.
Money: Five hundred dollars is a lot to spend on a dress.
Distance: Forty miles is a long way to walk in a day.
In the instances above, it seems reasonable native speakers would take the subject as a single entity since the complement is expressed as a singular noun. But how about the case of the complement being "enough"? Regrettably no grammar books around me mention about it. So I made a google survey on it
The results are:
X hours is (not) enough : 6,230+(12,600) X hours are (not) enough : 930+(805)
X dollars is (not) enough : 1,740+(1,280) X pieces are (not) enough : 276+(176)
X miles is (not) enough : 948+(813) X miles are (not) enough : 93+(19)
X pieces is (not) enough : 311+(113) X pieces are (not) enough : 308+(152)
X slices is (not) enough : 65+(2) X pieces are (not) enough : 37+(3)
As far as the unit is for 'duration', 'money' or 'distance', there is a tendency a singular verb is preferred. But in the case of 'pieces' and 'slices', nearly a half of people take 'X pieces/slices' as a singular entity and the other half take it as plural entities.
Thanks for the research, paco.

This is a close one.
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It is close. I would definitely use either.
You should say, "Two slices of pizza are enough for me." because slices is plural and you use "are" with plural nouns.