Since 'few' is used for countable nouns and 'less' is for uncountable nouns

Since 'few' is used for countable things and 'Less' is for uncountable things then why do we say;

I have less than 2 days/months/years.

?

Yes, time is an uncountable concept but we sure can count days, years, and months...

The phrase two days as in "two days left" can refer to a block of time, that is, to an amount of time remaining, rather than to the number of discrete individual days remaining, to their count.

We use few in situations where the number is relevant, and less in situations where the amount of something is remaining.

The semantics of the situation govern here.

Compare:

Give the child less mashed potatoes than you've given me.

Even though the noun "potatoes" is countable, in this context, semantically the reference is to a serving of the food, an amount, not a count.

TRomano's answer (and comments) explains everything quite correctly; however, we can also say in a few days' time as in

I'm going to have an exam in a few days' time

and in the following

May I ask a few questions?

substituting few with less would be incorrect. In this instance, "a few" is closer in meaning to "some".

As you adequately state it, the period can be considered both an uncountable chunk of time (as in the example you provide) or in relation to its individual units (hours, days). That's why

I have less than 2 days/months/years.

is correct. A similar use often occurs with money.

The new model costs less than ten pounds.

Regardless, we should also consider that for most native speakers fewer than would sound more formal than less than, which explains a higher occurence of the latter.

• Further details on this topic can be found in Cambridge Dictionary's blog entry by Liz Walter.
• Ngram shown here.