You should never mix more then a 1% solution of soap. But do be careful what you treat it with. You can have phototoxic results using dish soap, palms like fish tails can show damage. Also studies have shown tomatoes actually have lower yields using dish soap instead of insecticidal soap, the products are not the same.

Good luck.

Also you probably want to do a follow up treatment in 5 days.

The 1 percent solution of soap and water is abut 1 teaspoon of soap to 1 quart of water. I would not use dish detergents since it is the fatty acids in soap (detergents don't have those) that does the work.
Controlling spider mites can also be accopmplished by increasing the humuidity of the affected plant.

I use this spray on my banana plants--they used to get spider mites real bad in the greenhouse and it really works. It also got rid of the aphids on an elephant ear plant I got in a trade, so I just use it all the time now.

1 Tablespoon ammonia (cheap stuff from $ store, I used)
1 teaspoon dish detergent
2 1/2 gallons water

mix this together and spray the plants, top and bottom of leaves until they are good and wet. Repeat every five days for three weeks. It also works to green the plants back up because of the ammonia being nitrogen. I've used it every five days since I brought in my bananas, and haven't seen any spider mites at all in the greenhouse this year, and they were awful in there last year.

Still, the best and safest solution is to use a product that is made just for this purpose....commercial insecticidal soap.

In my experience insecticidal soap alone will not control an established spider mite infestation.