A 69-year-old man is prescribed ramipril 1.25mg OD for hypertension by his GP. Baseline blood tests before starting treatment were: Sodium 140 mmol/L, Potassium 4.2 mmol/L, Urea 6.0 mmol/L, Creatinine 90 µmol/L (eGFR >60). According to NICE guidelines, when should his renal function and electrolytes be re-checked after starting the ACE inhibitor?