Fiancee visa takes six months from the date it is being issued and it has multiple entry the validity of it is also being stated in your visa...no worries regarding that 28 days is applicable only for those under spouse visa for if you enter uk more than 28 after the issuance of spouse visa
it will affect the ILR application in the future as you need to stay or live in uk for within 24 months to get a permanent residency...
