Q:

Adjective for a period of time that feels much longer than it actually is

Is there a word for describing a period of time that feels much longer than it actually is?
To fill in the blanks below for example

Only few days have passed but it feels like months. How ___ .
The time we spent on the island has been ___ . It is as if we'd lived there for years.
Reading the source code is quite a ___ task.

A:

When time feels like it is passing slower than usual, we usually say that it is dragging:

2 (of time) pass slowly and tediously.

’the day dragged—eventually it was time for bed’
I don’t think it’s quite the single word you were looking for, but in conjunction with “time” it can fill the blank in all your examples.
It does have fairly negative connotations; we often used stretched in more positive situations. E.g.:

“The long summer days stretched out, so by the autumn it felt like we hadn’t been to school in years.”

