How to change the width of a given square wave with duty cycle 50% without using microcontroller or ramp solutions?
Buy one of these: -



I've used one exactly how you appear to be wanting to use one and it worked very well and saves a lot of hassle messing around getting component values right. Frequency is adjustable from low Hz to 1 MHz. Duty cycle variants exist that prevent 100% (flat line) or 0% flat lining.