This is a very curious but interesting question. IThe expiration date is the good time if the tablets are not crushed and well conserved (ie not in contact with liquids or meal )

The same time as an original package

in a big study even with american FDA they found most medicines do not go bad or remain effective although label is they have an expiery date although except for few liquids most nmedicines are usable and destroyed jusst for furtheri fb phama ceutical buaiess.

Since shelf life is determined by testing in the original container, the stability can be impacted as soon as the medicine is taken from the original container, eg highly hygrosopic medicines, soluble medicines, etc. Since generics will have different excipients (and therefore stability), I would imagine that a general rule relating to the active ingredient may not suffice. The only source of information I can think of is stability testing undertaken by the pharma company, which they are often hesitant to provide because it is not in line with their regulatory approval. There are usually local regulations which govern packaging of dosage administration aids, so they will be a good starting point. Also need to think about method of repackaging and timeframe in repackaged aid.

Depends on the nature of the tablet and the coating.

A film coated or specially coated tablet is relatively less vulnerable to atmospheric degradation. However, I guess the expiry should be within an hour or two of being exposed to the environment unless specific stability study in the similar or rather identical atmosphere has been carried out.

As a general rule, tablets lose 25% of their shelf life once they are removed from their original packaging.  This is given that they are repackaged into acceptable containers (e.g. polytops or dispensing bags).  So if a tablet's expiry date is 12 months from now and you repackage it, the new expiry date will be 9 months from now.