What is the idiom used when you don't do something right away but do it eventually?

Solution 1:

You could use “Better late than never”.

said when you think that it is better for someone or something to be late than never to arrive or to happen.

(Cambridge Dictionary)

Solution 2:

If the person feels guilty that things didn't go as planned, @Scott is right with 'better late than never'.

On the other hand, if the person felt the schedule was the problem in the first place, there's

(in) one's (own) sweet time

as slowly as one wants even though others want one to act more quickly

"You took your own sweet time turning this in."
"Well, [wildly implausible medical/familial/national-security excuse]."
"Yeah, well, you're still losing 20 points."