How far in advance do you need to apply for Visa I am UK citizen

I am visiting my daughter who is working there for 9 months, for a week then I hope to visit again next year for 2-3 weeks, I have read not to apply more than 3 months in advance because visa may no longer be valid,however I have also read that a visa is now valid for 2 years. What is cost of VISA if you apply yourself.