How long is a thousand generations?

Psalms 105:8 He hath remembered his covenant for ever, the word which he commanded to a thousand generations.

How many generations were there from Adam to Jesus? Less than 100?

How many generations were there from Jesus until now? Less than 50?

That would leave a lot of generations remaining.

Then considering the following:

Isaiah 65:20 There shall be no more thence an infant of days, nor an old man that hath not filled his days: for the child shall die an hundred years old; but the sinner being an hundred years old shall be accursed.

About how long would those remaining generations take?