Times

"Times" means occasions or instances.

I've met Richard on several occasions. I've met him several times.

At times

"At times", "now and then", "off and on" and from "time to time" all mean sometimes or occasionally.

'At times" I find myself wishing I had chosen another profession.

"Time and again" means "over and over", "again and again", repeatedly.

I've told you time and again that you have to send a copy of the monthly report to all the managers.

"Timely" means opportune, at a good moment, at an appropriate moment.

The advisor gave them some timely advice about the financial crisis and they decided to cancel the expansion plan.

"Over time" means a period of time in the past or in the future.

Over time they realized that they had made the wrong choice.

Over time we will see if strategy is the correct one.

"Over the years" means through time.

Over the years he discovered that what really made him happy was helping others.

"Once" is one time. "Twice" is two times. "Thrice", although quite rare, is three times.

He thinks he works twice as hard as everybody else.