Being granted a Spouse visa allows you entry to the UK for a period of 27 months. The UK Spouse Visa will be valid between the 'valid from' and 'valid until' dates. So you can enter UK anytime between those dates
The 'qualifying period' for application of Indefinite Leave to Remain (ILR) is 1 year and 48 weeks. If you leave your entry to UK too late to get this minimum period, you will have to extend your Spouse Visa at high cost, and then apply for ILR and pay again. Very expensive.