See What to expect in the first few days of broadband which says:
After your BT broadband has been turned on for the first time, it can take up to an hour or so to 'settle down'. In some cases, usually when you're a very long way from the exchange, it might take longer - sometimes up to 24 hours.During this settling in period you may experience an intermittent connection, usually accompanied by a flashing 'Broadband' light on your BT Home Hub. (If you're using another router or modem then the flashing light might be labelled 'DSL' or 'ADSL' instead.) This is all quite normal and doesn't mean there's anything wrong with your broadband. It's caused by your BT exchange automatically trying out different speeds to find out the fastest stable speed that your particular line can support. Once this has been worked out, then your broadband should become steady.
Although you should have a stable broadband connection after the first hour, tests actually go on for another 3 days to work out the final optimum speed for your phone line. During this time, you may notice your broadband speed keeps going up and down. Again, this is all quite normal and doesn't mean there's a problem.
So if you've had broadband for 3 days or less, you should only contact our helpdesk if you completely lose your broadband service for more than an hour.