A reasonable time will vary depending on how severe the repair issue is. For example, faulty electrical wiring is very dangerous and could pose a very serious risk to your safety, this means that your landlord should make repairs straight away, usually within 24 hours. This also applies to other issues that could damage health. However, for less severe issues your landlord does not need to act as quickly, although they should still make the repairs.
Electrical problems must be repaired by your landlord within a reasonable amount of time. Your landlord cannot make their own electrical repairs, they must have a qualified electrician to carry out any electrical work on their rental properties.