I've never done this, but I can think of a pretty easy way to do it, if you are content to measure the concentration of phosphate ion (PO43-), or phosphate+pyrophosphate, in the medium.

At various time points, centrifuge the culture to pellet bacteria and remaining insoluble phosphate. Prepare serial dilutions of the supernatant in water and measure the concentration of phosphate ion using any of the many versions of colorimetric molybdate-based phosphate detection reagents (easily prepared or purchased).

To also measure the concentration of pyrophosphate, precede the measurement step by including the enzyme inorganic pyrophosphatase (commercially available) to convert pyrophosphate ion to 2 phosphate ions.

If you are also interested in measuring organic phosphate, it requires first treating the supernatant with hot sulfuric acid and peroxide to convert organic phosphates into inorganic phosphate ion. This is a rather nasty procedure, best avoided if unnecessary for your purpose.

Thanks

I have a protocol fo phosphate determination using Malachite green and heptamolybdate. It was very sensitive (detecting impurities from glassware, but it reacted only with orthophosphate.