How long to cook roast lamb?

Does anyone have any idea of how long to cook a 1kg lamb roast? answers would be welcomed by my family!

Roast Lamb and HerbsPreheat oven to 225c
Long and slow cooking is best
Ingredients:
lamb roast, 1tbsp olive oil, juice of one lemon, salt and pepper, 1 tsp parsley, chopped, 1 tsp thyme, chopped,
1 tsp sage, chopped, 1 tsp sesame seed, 1/2 tsp mustard seed, 1 tbsp rosemary chopped, 1 clove garlic, 2 tsp mint, chopped. .
Method:
Wipe the lamb with a damp cloth. Rub the leg of lamb with the olive oil, lemon juice and then salt and the pepper. Combine the parsley, thyme, sage, sesame seed, mustard seed, rosemary, garlic and mint and pat this mixture on the leg of lamb, all over. .
Make a slit near the knuckle and place the garlic slivers into the slit. Sear in a hot (225c°) oven until brown.
Bake the lamb at 160c for 2 - 3 hours. The roasting time will depend on the size of lamb.
When it's tender, put on hot serving platter to rest and make gravy from the pan...
Add little water to pan and scrape up all the juices and baked pieces, bring to a boil. Now add some flour and water that has been mixed to a smooth paste and let it boil for a minute or two. You can use cornflour and water.
Hints: The two herbs that are synonymous with lamb are rosemary and mint
(a)I chop all the herbs and place with seeds in mortar and pestle and grind to fine powder. Spread out on board and roll leg to coat.
(b)Place the lamb leg on a bed of rosemary sprigs in roasting dish. Blends a magic flavour to the roast , veges and gravy when sprigs removed...
(c)Mint saucethe perfect compliment to a lamb roast
(d)Next day ~ super fresh bread with lashings of buttercold lamb slices . . salt and fresh ground pepper and a slosh of mint sauce.
(f)Add a teaspoon of marmite to the gravy.

Thanks fisher! That sounds great

Usually 20 -30 mins per 500g.