Can You Tell From Your Prints It's Time For A New Batch Of Developer Or From The Developer Itself?

I use batches of developer (Ilford Multigrade) multiple times with good results, even when it starts looking orange or amber. 'Comes a time when prints have an odd "faded" look about them, and wonder if developer has become "weak", and should be dumped down the drain. Yet I've had satisfactory results even when it bocomes that amber color. What ages or weakens developer - of course it's the using/reusing, but exposure to air? Heating each time for re-use?

No, FB paper is expensive..... re-doing prints costs both time & money. I don't re-use chemicals and I am careful to observe the amount of prints i run through. That goes both for developer & fixer.

12 prints to a tray..then refresh

With any developer, if the print starts taking too long to develop, I put the print in a tray of water, dump the developer, mix new developer, refill the washed tray and put the print back into the new developer.

Yes, if I'm using factorial print development:
Factorial Development

I use developers that can be used the next day by covering the trays w/ saran wrap. There's several that allow that, so that cuts costs down a little.

The developers will tell you the maximum number of prints that's possible. I don't keep records, put do count the prints that had been in the soup at the end of a session.

15, 11x14 prints is about my limit for 1 night of printing, and less for FB because of the longer wash times and longer times in the developer. It doesn't pay to stretch things beyond the developer's recommendations.

Don't ask me why I know that :[

What GregY said + 1. I would say the advantages of using developer always within the manufacturer's recommended limits are (1) a fresh crisp image; (2) repeatability.

If your reason for using beyond that limit is budget, you might consider a home-brew, perhaps? Or consider buying 5 litres MG developer at a time, which is much cheaper. Then on opening, re-bottle it into smaller amounts, with protective gas. For example, I bought a set of 500 ml and 20ml glass bottles. I divide 5 litres into 8 x 500 ml bottles and 5 x 200 ml bottles. When the small bottles are all used, I open another 500 ml bottle and sub-divide that. I got the tip from a long thread about protective gas on the FADU forum here. I am still working my way through the first 5 litres, but so far the bottles have kept perfectly.

you can test your developer by taking a scrap of paper and make a test strip with no negative in the enlarger and develop it, you can also test your developer by taking a coin and putting it on your scrap of paper and turn the room lights on then develop it and see if you get blacks. life's too short to use spent chemicals, and they usually aren't too expensive ... I get sprint by the cube it's cheap..