How to know game lenght?

When you make a move, you get 10 seconds added to your clock. When your opponent makes a move, they get 10 seconds added to their clock. That happens after every move. 

If you assume an average game of 40 moves, the game could last over 40 minutes if each players used most of the time on the clock. In many cases, the games will end much faster.

Martin_Stahl  Thank you Sir

Anybody can  answer my first Question too?

1.How to see the Total length of each game I have played in chess.com?

Not sure exactly what you are asking. You can see the total number of moves for each game in your archive, which you can filter by game type. I don't believe you can find get more granular than that with site tools. You an download games with timestamps and can figure out how much time you spent on moves, if that is what you are asking, but that would take some work to massage the data.

If you want to know the Total Length of each game you play on here, you have to calculate it out. 

I'll give you an example - because I calculate the exact time used in my OTB games.

Let's take your 51 move game on Feb. 7.

You started with 15 Mins. and had 4-14 left - so you used 10 Mins 46 Secs.  Plus you made 51 moves at 10 Secs per move - which is 510 Secs  = 8 Mins 30 Secs.

 You then add those 2 Times together - so 10 Mins 46 Secs +8 Mins 30 Secs = 19 Mins 16 Secs. So that is the Total Time you used for that game.

And your opponent used18 Mins 50 Secs. 

AussieRookie Thank you this is what I was looking for.

Martin_Stahl thank you. Aussie answered it. I was looking to find how much time I spend for a game

Including opponent time.

I recently made a tool which calculator monthly and yearly archives of time spent on chess.com

https://chesstime.netlify.app

nice tool, really helpful
one thing, you need to fix the issue with asking for a year that hasn't finished, as 2024
calculate until the actual date and just show the months until the day calculated
cheers!