Use a constant current source to charge a capacitor; compare the capacitor voltage to the reference voltage (0.05-2V); use a switch (2N3904, 2N7000, etc.?) to discharge the capacitor. Use logic between input and compare signals to obtain your monostable (i.e., output = input low && comparator low).
The current sources and capacitors need to be matched for equal time constants; if they need to be better than 5% matched, you probably want a calibration step (lengthy!) and some low range trimmers (5-10% range) per CCS output.
Don't bother with 555s. They're terrible, and not suitable for long time constants like this anyway. Attempts to force them into such applications result in such hacks as, well..
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
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To be fair, PWM into a timer is interesting, but you probably won't be able to get the design range and linearity quite right. You're also subject to the leakage of the diode (~uA) from the control line (which will be significant against 100k ohms and a long time constant like this), which doesn't have a good solution (a lower leakage diode, like a diode-strapped BJT (C-B junction) would be better -- transistors have much less leakage than generic switching diodes -- but it's still something of a hack).
I'll have to remember that though... suppose you PWM'd a variable current into your timer. Now you have two joint variables, and the result is the product of them. So if you needed a variable time based on the multiplication of two signals, this would be one way to do it.
Your circuit might not be best for its intended purpose, but it connects with some interesting analog math functions than can be quite useful for certain purposes!
Note that the PWM clock period must be N times faster than the minimum time period, for 1/N accuracy. Since it can only tick over in quantized steps, so you need N steps per duration to get that granularity.
Which has another valuable feature: you can use a similar circuit as a clock divider or counter: yes, a counter, in analog parts, using a few orders of magnitude fewer transistors than the equivalent digital process (e.g., a chain of T flip-flops).
Tim