I picked this 1959 1/2 HP 155.7575 up at an estate sale last week for $40 including the homemade farming disc stand. I'm in the process of researching and restoring it.
Someone made a comment about a 3-minute spin down and inferred that anything less than 2 minutes of spin down time indicated the bearings should be changed. I'm fairly sure mine has not been used in a long time and it spun down in about 17 seconds with the stone and wire wheels on. After I removed them--and a couple of big washers on the wire side that were doing nothing except causing vibrations, the bare shafts stop in 27 seconds after about a full minute run up. After performing this test multiple times the time increases a little but nothing over 30 seconds.
The thing that has me scratching my head is even after doing this over and over, I put my fingers on the bearings and they are completely cool. So is 30 seconds about right or do I need to change bearings?
You're fine and please don't listen to that 2 minute plus rule, it's rubbish.
I have had over 8-10 of these same grinders in various condition (including new old stock) and never had any of them spin more than a 30 seconds at most, including my three very nice Baldor buffers and grinders.
Cajunfirehawk in this thread is the best one to ask, he has a load of these in excellent condition.
EDIT: Read exmaxima1 post above.
One way to tell if the bearings are bad by the sound of machine while spinning especially when turning it on and off. Do you hear any whine and metal sounds? When listening to grinder when turning on, does it turn on, spin and then quickly settle to a smooth constant sound? Slightly bent spindles can also cause noise and bad performance and ruin bearings.
When turned off you can also turn the spindles close to the bearings with your hand and feel for roughness in bearings while very slowly turning them. Feeling bearings after use for excessive heat is also very good idea like you said but bearings and spindle can heat up under normal conditions.
By your description, it sounds like you have a nice grinder. Use it and enjoy.
Now, it's very normal for large (14", more like 20"+) disc sanders to do this like an Apex and other large industrial disc sanders and can have a brake on them to make them stop spinning for safety purposes as they can spin for over 10-20 minutes after power is off.
PS> Your cast iron base and tool rests on that grinder are very desired. Nice find.
, that is one area of block info you will not get from our fearless block ring leader, kinda like where Hoffa is buried 
