What is the spacing of obstacle lights on towers?
In theory you might be able to make an estimate, but practically speaking it's impossible and a lot easier to just look at a sectional chart.

The FAA has a detailed guide on obstacle lighting standards, which usually applies to structures over 200ft in height but there's a lot of possible variation:


  Recommendations on marking and/or lighting structures can vary
  depending on terrain features, weather patterns, geographic location,
  and in the case of wind turbines, number of structures and overall
  layout of design.


The actual number of lights and their placement depends on the type and size of the structure. Appendix 1 of the FAA guide has examples for water towers, radio masts, cooling stacks, bridges, chimneys etc. The vertical light spacing seems to vary from 250-375ft, depending on the structure type, but I didn't go through it systematically.

So if you have plenty of time you might be able to determine the approximate height of a structure by comparing its lighting layout to the guide and estimating based on that, but that's very unlikely to give you an accurate number. For a pilot in flight or planning a flight, that isn't safe or practical and the simple solution is to just look at the VFR sectional chart to get the height. For example, this tower is 1094ft high and the top is at 1994ft above sea level (so the ground level is 900ft above sea level at that location):