Most of the comments above answers your question, but the terminology used here is not correct. Agarose gel don't polymerize but rather solidify (non covalent interaction). It's only the polyacyrlamide gel which forms by polymerization (covalent bonds).

What is your point exactly?

When agarose be polymerized it is quietly obvious.

It is true you need 0.8% agarose gel running 80v for 40 min, but ethidium bromide is carcinogenic and not to be used. You can use safe stains.

It depends a bit on the size and thickness of your gel.  Smaller (~100 ml) gels should be set in about 20 minutes.  Larger will take longer.  The gel will look more opaque as it polymerizes.  You can test if it's polymerized by gently touching a corner of the gel.  It should feel firm and not jiggle in the center.

Ignore Alreza.  You can use what ever stain your labs typically uses.

Hi

It usually depends upon temperature, concentration and quality of your agarose. low melting agarose usually takes more time to polymerize, but all these don't  matter if you follow these steps. 

1- Let your agarose boil. If you are worrying about evaporation and increasing the concentration you can add a slight amount of water before to boiling and let the excess water to be evaporated.     

2- Wait until the temperature reaches 40-50C (check by touching; it shouldn't hurt after several seconds) pour agarose into the tray.     

3- After 10-20 minutes check the gel if coagulated by one gentle bending. Before placing in tank you can pour on it some cold working TAE buffer to ensure that your gel is thoroughly tightened and to have good wells.         

4- Place the tray into the tank.        

Dear Amith,

The concentration of agaraose depends on DNA Size, Usually 1 to 2% gels are used for detecting plasmids (several kb long) or their fragments.

The agaraose polymerization time dpends on concentration of agarsoe.

High concentration gel solidifies faster than low concentration, its usually 15 to 20 minutes.

Attached file may help you

Hello, Amit. 

It usually takes 20-30 minutes for a gel to solidify. But I prepare gels by batch, so I leave them for an hour. If you're using Sybr safe as the stain, make sure you cover the gel with paper because the stain is light-sensitive. If you're to use EtBr, no need to cover it while letting it solidify. Good luck.

Regards,

NRB 

Agarose solution set quickly (double-helix formation), but undergo a slow conformational change (rope formation) after they have solidified. Therefore, for defined pore size, store them over night in the fridge before using them.

It depend upon percentage of Agarose. It will take around 5 -10 minutes only.

It usually takes 10-20 minutes.

Most of the comments above answers your question, but the terminology used here is not correct. Agarose gel don't polymerize but rather solidify (non covalent interaction). It's only the polyacyrlamide gel which forms by polymerization (covalent bonds).

I like to let mone cool at 4c it will set in 20 minutes (100ml gel), on the bench 30 to 45 min.  Depends on size of gel casting. 

Best

Agarose melts around 90, but sets around 40 °C. However, the resulting gel is not final, a conformational change (supercoiling)  takes over night in the fridge