You need to think about pore size of the gel, and EtBr as well. As per my experience, you should prepare the gel just before 2-3 hours of loading the gel with your sample.

If you keep for a long time then, probably the wells get dried and while running you may see some of the wells, your nucleic acid is still sitting in the well and not able to enter through the gel. If you keep the gel for a short time, its okay, but I don't advise to keep in the buffer for a whole day. 

Generally, I advise my students to start gel preparation when they finish their PCR,  

(Hi Annette)

forever and ever and ever. probably not but at least for a week or so if you put it in the fridge and prevent it from drying out (in running buffer, not water). we always made them in bulk to use over the next 2 weeks. you can add etbr later to the bottom running buffer chamber if you prefer.

Hi Annette,

     Keep the gel in running buffer inside 4degree or -20degree fridge after it is solidified

you can buy gels that last weeks, so no problem for gels made by yourself.

Tian Xia is correct,

For keeping constant data on gels, its good option to buy gels that last weeks, so you don't have to worry about any variations you might come across if you store gels in buffer in either 4 degrees or some says you can even keep in -20 (?). 

Then all the time you need to keep conditions constant which gel you stored for weeks or days, what buffer you used, whether that was fresh or old......

Either keep everything fresh or get ready to use gel as Tian says. 

Hannes is also correct, they are usually doing that means like them, prepare at once and use as batch, but adding EtBr in tank buffer that means more exposing to more carcinogen. Also, during electrophoresis, EtBr in gel gets entrapped in to sample and that is how even in less concentration is gel, you can see bright illumination of EtBr under UV.  

We always use EtBr in gel because very less quantity you need, and that also gives you uniformity throughout the gel, some experiences bioleaching of EtBr if you keep in buffer. I experiences whenever we use less quantity of EtBr, (10 to 15 ul of 1 mg/mL) i.e. 200 to 300 ng/ml concentration of EtBr in 50 ml gel, the next day, that fades. So, we always use EtBr in gel. i.e. keeping this carcinogen under 500 ng/ml for lab safety.

Thanks for your replies! I will keep the gel over-night then, and be careful to use the same conditions next time I repeat the experiment.

We are actually not using EtBr anymore, but a less toxic reagent called "Nancy". Hopefully, it will stay in the gel overnight. I'll try to keep the gel in such a small volume TEA as possible, to prevent unnecessary leakage.

Hello Annette E. Sköld,

I am wonder whether or not your gel could be saved for about a week and using the safe dye for staining the DNA instead of the EtBr?

Because I am about to do the same (saving the gel for a week before running or loading the sample, only prepare it and using Safe Dye for staining).

Thanks in advance.

I just ran a gel that I had in the 4degree for over a week just wrapped in foil (gel left in it's plastic rack and then the whole thing wrapped in foil - foil does not touch gel). The gel came out beautifully. We use EtBr directly in our gel. All was fine. I don't store in buffer (just make sure there are no holes in whatever wrap you use so the gel doesn't get exposed to air and dry out). I regularly use gels stored this way but this is the first time I have left one this long. Still it wasn't a problem at all.