Seems to vary a little depending on which method you use. From internet:
Sterilising to make it safe
There are several ways to sterilise feeding equipment. You can either use a large stainless steel saucepan to boil things or buy a special sterilising unit.
Boiling Fill a large stainless steel saucepan with cold water. It must be large enough for all feeding equipment to be totally immersed. Immerse feeding equipment in the water, ensuring that there are no air pockets trapped in bottles, teats or caps. Cover saucepan with a lid. Bring to the boil. Once water is boiling, continue to boil for at least 5 minutes, ensuring the pan does not boil dry. Allow to cool. Keep pan covered until bottles and teats are needed. Use this pan only for sterilising your feeding equipment, not for normal cooking. If you are not going to use the bottles within 3 hours, then they should be re-sterilised before use.
Sterilising units You can buy sterilising units from department stores, Mothercare and larger Boots stores, and prices range from ú10-ú40 (?25-?70). There are 3 main types of sterilising unit. They are all effective at killing harmful bacteria when used according to the manufacturer's instructions. It's a good idea to practise using your steriliser before your baby is born so that you can get used to how it works. Whichever unit you use, always wash your hands before removing the sterilised equipment. Do not remove teats with your hands - use special tongs and remember to sterilise these before using.
Cold water sterilisers ((ú15-ú25 / ?24-?30) These consist of a large container with a plunger and lid. The container is filled with cold water, and sterilising tablets or solution are added. Bottles, teats etc are then fully submerged in the water, ensuring there are no air pockets or bubbles in them. Keep all equipment submerged by placing the float tray on top of the equipment. Sterilising takes 30 minutes, and items in the solution will remain sterilised for up to 24 hours as long as you do not touch or remove them from the water. After this time, you should discard the old solution, rinse the steriliser unit out with tap water, and then refill with new sterilising solution. Shake off excess solution or rinse equipment in cooled boiled water before use. If you are using a metal knife to level scoops of infant formula, you will need to sterilise it separately, or use a plastic one that can be cold water sterilised.
Steam sterilisers (ú30-ú40 / ?65-?69) These are free-standing electrical units. Bottles, teats and any other items you wish to sterilise are placed inside, together with a specified amount of water (the instructions will tell you how much). You then replace the lid and switch on. Sterilisation takes about 8-10 minutes, after which time the unit will switch off automatically. The steriliser should then be allowed to cool for 2-3 minutes before you open it because of the hot steam that will escape when the lid is lifted. If left unopened, the bottles inside will remain sterile for around 3 hours. If you're not going to use all the bottles at once, you will need to re-sterilise the remainder later.
Microwave sterilisers (ú12-ú20 / ?27-?32) These are special units that act as steam sterilisers when you put them inside the microwave. The instructions will tell you how much water to add and how to arrange the bottles in the unit. It takes about 10 minutes to sterilise, and a further 2-3 minutes to cool down. It will keep items sterilised for 2-3 hours as long as the lid isn't lifted. If you're not going to use all the bottles at once, you will need to re-sterilise the remainder later. Microwave sterilisers are cheaper than plug-in sterilisers but you will need to check that the unit fits inside your microwave before buying. Remember, you cannot put metal in the microwave, so you will need to sterilise the knife used to level scoops of infant formula separately, or use a plastic one that can go in the microwave steriliser.