In the UK, fleas can be a year-round problem for both indoor and outdoor cats. We as humans can carry fleas (adults, larvae and eggs) from the outdoor environment and deposit them within the home environment.
For effective control, adult fleas on the cat must be killed and re-infestation from the environment prevented with regular application of veterinary prescription-strength spot-on treatments and environmental control (frequent vacuuming and flea sprays).
Once adult and immature fleas have been completely eradicated from a household environment, further controls can be reappraised. In a household where none of the pets go outside, the frequency of treatment may be reduced. However, if your cats venture outside or you're in contact with animals who do not receive regular flea medication, further treatment will invariably be needed to prevent re-infestation, probably in the form of a single product applied monthly.
An 'on-off' approach to flea control (treating intermittently) is not recommended as this may provide ideal conditions for the development of flea allergic skin disease in susceptible animals and is an ineffective approach to establishing control over the flea population.