Till/Up to

'til = until. Up to the end or All the way to the end or 'Til the end. You can make them interchangeable or not: 'Til the end -can focus on the aspect of release at the end whereas 'up to' focuses on any tension along the way towards the end.
EX: He was a great room mate up to the end, all the way to the end of his stay). VS. She was patient 'til 5 more people stood in front of her in line

I am more confused than before

Can you please explain it in simple words?

Thanks

Are 'till' (= until) and 'up to' interchangeable?-- Very often yes, they mean the same and are both prepositions. But as you know, English has many exceptions, so I cannot guarantee that they are always synonymous. Do you have a particular sentence that you wish us to examine?

'til = until. They are pretty much interchangeable. However, if 'the end' is important you might use until, but if the time that is past while reaching the end you would probably use up to the end.

"He was happy until she died" => It's important that he wasn't anymore after she died
"He was happy up to the end" => It's important that all the way through, he stayed happy.

@Mister Micawber
You are always great

Till what time you would stay?
Up to what time you would stay?

Are t

Till what time you would stay?
Up to what time you would stay?

Yes, they are both grammatically OK and synonymous. The context makes ''till' the right choice for register.

grammatically OK - This OK makes me rethink about my sentences. Could you please tell me what would be the correct way to ask this? I mean which is grammatically GOOD not OK?

@Anonymous
Thank you, too

Once again thank you !
If the sentences are grammatically Ok, what would be correct way of asking it? I mean which can be grammatically GOOD ?

Best,

They are both grammatically good. However, it would be more normal to use 'til in this sentence. So up to would sound a bit weird, even though it's officially correct.