How long should I let a tea with lemongrass steep

There is an ISO standard for tea preparation.

To quote the summary:

• The pot should be white porcelain or glazed earthenware and have a partly serrated edge. It should have a lid that fits loosely inside the pot.
• If a large pot is used, it should hold a maximum of 310 ml (±8 ml) and must weigh 200 g (±10 g).
• If a small pot is used, it should hold a maximum of 150 ml (±4 ml) and must weigh 118 g (±10 g).
• 2 grams of tea (measured to ±2% accuracy) per 100 ml boiling water is placed into the pot.
• Freshly boiling water is poured into the pot to within 4-6 mm of the brim. Allow 20 seconds for water to cool.
• The water should be similar to the drinking water where the tea will be consumed
• Brewing time is six minutes.
• The brewed tea is then poured into a white porcelain or glazed earthenware bowl.
• If a large bowl is used, it must have a capacity of 380 ml and weigh 200 g (±20 g)
• If a small bowl is used, it must have a capacity of 200 ml and weigh 105 g (±20 g)
• If the test involves milk, then it can be added before or after pouring the infused tea.
• Milk added after the pouring of tea is best tasted when the liquid is between 65 - 80 °C.
• 5 ml of milk for the large bowl, or 2.5 ml for the small bowl, is used.

Green tea is not to everyone's taste. You can try adding honey instead of sugar. As honey has more of a distinct flavour than just sugar it may help mask the green tea flavour more successfully. Another suggestion is to add some mint leaves if you're a fan of mint. If you're drinking green tea solely for health reasons the mint can be excellent for aiding digestion.

Certain tea companies also sell 'light' green tea. This can be more palatable, especially for people like yourself who don't like the taste of steeped green tea so changing your brand of tea might be helpful.

It may be that with perseverance you may acquire a taste for green tea (I personally used to hate camomile tea but continued to drink it every day and now it's my favourite).