A Racor 500 is a proprietary configuration .. and of course comes with a proprietary price. It uses a 'resinated' technical filter paper for its filtration media. There is only one worlds source for such 'paper', so you can easily find the same media in cheaper 'industrial' filter cartridges. For a recirc. system I'd opt for melt-blown 'graded pore density' polypropylene, etc. 'depth' filter cartridges.
In a recirc system the filter and its retention rating isnt 'that' important, its the 'number of turnovers' that result in clean fuel (a mathematical 'trick'). You could use compressed pubic hair to filter, it would just take a bit longer to get to the acceptable tank particle levels.

As regards 'run time', its best to run the recirc. any time the engine is 'on'. That way you're not removing 'particle storms' that break loose from tank walls during 'heavy weather' but rather are constantly removing the 'seed particles' and fungal, etc. spores which are the 'sites' upon which such particles 'grow' (larger and larger) and thus tend to maintain the tank internals in a somewhat 'pristine' condition at all times. Obviously you would want to run the system just after taking on fuel especially from low turnover sources.

A good way to 'buy' fuel is first fill a small clear glass container with the oil, hold it up between your eyeballs and strong light, and denote if its 'cloudy' or has a haze. If it has a 'haze', then its already contaminated (~5µM and above) ... so close down and move to another source to get your fuel from a highturnover source to get 'fresh' fuel that isnt so contaminated. Do this and you wont NEED to so depend on 'filters'.

Lastly, all filter positions should be equipped with vacuum/pressure gages ... so you know WHEN to change out the filters instead of waiting for a storm to enjoy 'power-puking' into the bilge while doing a filter change. You can apply small 12vdc 'differential alarm switches' with adjustable 'set-points' across each filter housing and wired to an 'alarm'.
A small 2+ gallon gravity feed 'day tank' that contains already filtered fuel and 'all ready to go' is damn good insurance .... when you begin to 'fuel starve', or lose a lift pump, just quickly throw 2 valves and youll have about 2+ hours of fuel - Long enough to 'get out of trouble' and to sort things out 'later-. The most important aspect of fuel filtration, etc. is to start with a CLEAN tank.