Learn English – Past/after when talking about time

Morning needs an article, so your example

I messaged you in the morning.

But since this is past tense, it would be better to write:

I messaged you this morning.

For the future it is OK though:

I will message you in the morning.

Not a very

• long period of time
• long time span

And of course short as the opposite of long.

Sometimes

• window of opportunity is used meaning

a short period of time that is available for a particular activity (LDOCE)

but without more context I'm not sure it fits your situation.

Or one can just quantify time as an uncountable noun:

• we won't have much time (look for definition 5)
• there won't be a lot of time