For a press commissioned within the last 20 years, then you should have a set of electrical inspection and test documentation in accordance with EN 60204-1, and the OEM data.
BS7671 & the IET "PAT" guidance is irrelevant to this scenario.
An IR test is a very broad brush, and cannot be applied in many ways to a complete power press in a similar way it can to a kettle or lighting circuit in a property.
Additionally the requirements for power press control systems these days are much advanced in relation to that guidance, the pictures shown in the HSE document are woefully outdated already.
You should be doing thorough examinations, and the person doing these must be suitably electrically competent if they are looking at the electrical power and control systems on the press.
The IR of a piece of equipment does not change overnight.
If the equipment is correctly designed, constructed and maintained, with suitable I&T following any maintenance intervention.
Any reduction in IR values in the relevant areas would be indicated.
A reduction in IR, should also not be in a position to cause a hazard to anyone interacting with the press.
If for example the motor IR reaches a value low enough for sufficient current to flow this would be required to trigger the circuit protection, and any earthing or bonding (which are two completely different things, even though they are both protective measures) must be in place to prevent the hazard.
An adequate PUWER assessment and adequate previous thorough examinations of the electrical power and control systems of the machinery must have highlighted any deficiencies, and by now for compliance, these must have been remedied, therefore the risk is very low.
The time interval cannot be defined without intimate knowledge of the equipment, its maintenance, environment and use.
Some I would be putting at the most, annual, others, potentially up to every 10 years.
The details of the machine, the environment and the utilisation all go to determine this.
Only the electrically competent specialists who deal with your actual presses can give you this information.
Though you want to make sure that they are competent to do this before you rely on it.
Not all would be in my experience.