Here in China, it is normal to reuse tea again and again. Typically the first cup from the tea is bitter. Chinese people commonly pour one cup and then throw it away and drink the second cup from the same leaves. Workers here can be seen with a large mug or jar of tea. When drunk, they top up the water again with hot (but not boiling) water. This will go on all day.
The next day, however, they will not reuse the tea. My Chinese wife always tells me off if I reuse the previous days tea leaves. She says they are bad for you, though I have never had any problem myself.
Hmmm.....
Well, I've lived in India for 12 yrs & this is how I make 'masala chai' or 'spiced milk tea' at home & in my restaurant-
2 cups water
1 teaspoon black tea, loose leaf (I use Tokla tea from Nepal)
1 tablespoon grated fresh ginger
10 black peppercorns, coarsely ground
500 ml full fat 'whole' milk
Sugar to taste
In a stainless steel 3L pot combine tea, water, ginger & black pepper, bring to boil over high heat. Allow to boil for about 2 minutes.
Add milk to boiling tea, again allow to come to a boil. (Watch the pot closely in case the milk begins to foam, if it does foam- stir & decrease heat a little) Allow to simmer uncovered for 6 minutes.
Remove from heat, let stand for 2 minutes. Add sugar to taste if desired & stir. Use a tea strainer to strain into cups to serve. (or serve sugar on the side so people can add sugar to their own tastes)
Some notes-
I think you are trying to make 'tulsi' chai?
Tulsi = Indian holy basil
If so usually the fresh tulsi leaves (around 8-10) are added to the boiling tea mixture with around 8 green cardamoms, 2 cloves, 1/4 tsp fennel seeds, and a pinch of cumin seeds as I recall. Some Indians dry roast the whole spices to mellow the flavor a bit.
Use spices like fennel, cloves, star anise, cumin sparingly as their strong flavors can take over & give the chai a 'mouthwash' taste which isn't pleasant.
Best Answer
The official answer from your health department is no, it's not safe. After four hours at room temperature, you have to pitch it.
That standard is a little on the paranoid side. Which is appropriate, since they have to protect people for whom a little salmonella is potentially life-threatening (the infirm, babies, the elderly, etc.) If you're young and healthy, the odds are it will do you no harm whatsoever. Even a glass of milk out overnight is merely going to be sour rather than toxic.
So, it's up to you. I'd say it's a cup of tea and it will taste better if you make a fresh one, even if the risk to your health is small-but-not-zero.