Heathrow Terminal 5 sees its heaviest traffic on weekday mornings and early evenings, when the M4, A4 and M25 are shared between commuters and airport travellers. If your journey falls in one of those windows, building in some extra time is sensible; outside them, and at weekends, the roads are typically quieter. Either way, the route is planned around the traffic on the day, not around a fixed schedule that ignores it.
The pickup point is settled in advance so there is no confusion on the day, and any airport leg is flight-tracked, meaning the car follows your real landing time rather than the original schedule. Either side of the drive, expect the usual airport process — check-in and security outbound, or passport control and baggage reclaim inbound — both allowed for in the included waiting time.
A quick note on group size and luggage at booking saves any surprise on the day: a saloon is comfortable for four with two large cases, and an estate, MPV or 8-seater takes the extra bags, buggies or kit that a bigger trip brings. Since school holidays and summer tend to be busier for journeys like this, booking a little ahead makes securing the right vehicle straightforward.