never use neat coolant as it will have very poor heat transfer capability, always cut with distilled water.
the water does the cooling job, the glycol formulation does the protection and management of the freeze and boil of the water in the coolant mixture.
ergo - always use the proper recommended ratio.
when using a regular OAT, HOAT coolant - replace it every 4 years or 100,000 kms. The toyota red and pink are really good products, the best ive used is BASF glysantin.