Beckhoff Twincat - set timer time manually

Hi,
I've been working with Beckhoff TwinCAT for a small time now and I already know the basics. There are however few things I can't make out so I'm asking your help.

I need a timer which time I should be able to set manually while the program is running.
I made a button to the visualizations page and by pressing that button it opens a numpad.
So I need some kind of timer which starts when a BOOL variable goes on (TRUE) and it runs the time I've set to it and then sets another BOOL variable on.
For example if I press the button on the visualizations page and enter number 500 using the numpad which opens, then when the timer starts it runs 500 milliseconds and then sets the another BOOL as TRUE.

By the way I'm using 1 millisecond in PLC task cycle time. Should I use that cycle time somehow when building the timer/counter? For example making a function and using FOR loop or something?

You can do this with a TON timer
what langauge are you using LD or ST or another.

here you go unzip and load it.
do not mind errors from libs and targets.

Thank you shooter very much. You helped me a lot.
The program you sent was just what I was looking for.

For similar tasks, I have used both the TON function block and just monitored elapsed time on my own using the GETSYSTEMTIME() call (or similar). The later takes a little effort because it returns two 32-bit integers that you must combine. This will surely be easier when one can use 64-bit integers in PLC code. Perhaps one can in TwinCAT3 (my next task).

The TON function is traditional in PLC coding. I recall having to play with it a bit to get the timer to reset. I vaguely recall that it only allows 1 sec increments, so check that.

time is a simple dword (32 bits) in version 2
and yes it does have a problem in 49 days.