how do you know when to get power of attorney?

You really need to do it before you need it, iyswim? As has been said, your mum actually "grants" you poa...if you wait until she is no longer considered able to do that, then you have to go to court for a Guardianship order. (I am in Scotland, it may be different elsewhere in UK).
It isn't that expensive and once you have the completed paperwork you can just put it away until such time as you need to use it.

Hi ssd. Yes, get it done now. It does cost - possibly up to £1000 (I am a Social Worker for older people so really should be able to be more specific ) But it's money well spent and will give you both security - you, from allegations of financial abuse, and she will know that everything's being deaslt with and in safe, trustworthy hands.

If you leave it 'til she no longer has Mental Capacity, you would have to go through the Court of Protection and that can take months - not good when there are bills that need paying. But setting up a POA is a relatively quick and simple process I believe.

Sorry, I cannot link to the stuff I have - it is all relating to Scottish Law, so may not apply to you anyway.

This explains a bit about it

We have just looked into this for my MIL.

Do it sooner rather than later.
You don't 'get' PoA, she has to grant it, so has to have the cognitive ability to understand what she is granting.
This is much, much easier to do while she is well and hopefully happily willing to go with it.

The cost is about £250, I think. Any solicitor can help with it.
Give me a minute and I'll dig out the initial info we were given.

Once the PoA is in place, you do not have to use it, unless there is a need, so getting it done sooner rather than later is a good idea.

Once a person does no longer have 'capacity' to make these decisions, a guardianship has to be arranged which is much more complicated and involves other agencies.

I think in your situation I would get in - and its considered best practice to get somethings arranged ahead of time
This link looks useful:

www.ageuk.org.uk/money-matters/legal-issues/powers-of-attorney/

Not sure about cost, but you MUST do it when your mum still has her faculties. Get it done now, then use it when you need to. If you wait until her mental capacity is reduced, you'll need to get welfare/financial guardianship which is a far more expensive and cimplicated process.