Your question about when it makes sense to stop paying in depends on three things:
- The possible growth rates of your investments.
- The costs of going over the LTA.
- The benefits when under the LTA, which translate into lost opportunity to benefit if you undershoot.
It's really hard to make a good guess about growth rates, so I won't try, and instead focus on the other two points.
Going over the LTA
Firstly, actually going over the LTA isn't necessarily that bad, depending on your marginal tax rates at the time of paying in and the time of taking the money out. One quite common case will be that you are a 40% taxpayer now, and you would be a 20% taxpayer when you take the money out.
Then if you pay in £1 now, it costs you 60p from your after-tax salary. Suppose it grows by some factor due to investment growth. The actual number doesn't matter, so pretend it's 3, i.e. you have £3 when you come to take the money out, and it's all over the LTA. Then you pay 25% penalty leaving you with £2.25, and then 20% tax on the remainder which is 45p, leaving you with £1.80. That's exactly what you'd have if you'd taken the 60p net pay, put it in an ISA and invested it the same way.
In most cases paying the 55% penalty for a lump sum doesn't make sense: it's equivalent to paying the 25% penalty and then 40% income tax. Given the 2016 pension freedoms, you can also get a "lump sum" by just using normal income drawdown. If you're so rich in retirement that you're earning over £100K then perhaps the 55% penalty is a better choice :-)
Things look worse if you are a 20% taxpayer now, or would be a 40% taxpayer when taking the money out.
On the other hand, they look better if you are a 45% taxpayer now, or you are stuck in one of the weird marginal tax bands between £50,000 and £60,000 if you are subject to the child benefit charge, and between £100,000 and ~£125,000 where the personal allowance is withdrawn. Because of the effects of the extra charges, your marginal tax rate is higher than 40% so it often makes sense to make pension contributions to get your taxable income down to the bottom of those bands.
Another possible mitigating factor for the LTA is the timings of the tests against the allowance. This is really complicated and I'm still not sure I've got the rules right. But I think that if your pension reaches the LTA after you reach minimum retirement age (likely to be around age 58 for you - from 2028 the government is planning for it to be your state pension age minus 10), then you can immediately stop paying into your pension and put it into drawdown. Then as long as you withdraw any future growth by age 75, keeping the balance under the LTA, you won't get charged any penalties, if I've understood the rules correctly. I talked about them a bit more in this answer.
Pension benefits when under the LTA
For any money under the lifetime allowance, you can take 25% tax-free, and the rest is taxed as income. So for example using the same example as above, a £1 gross contribution becomes £3. You get 75p tax free and pay 20% = 45p in tax on the other £2.25, leaving you with £2.55 instead of the £1.80 you'd have from an ISA. That's a gain of 5/12ths (~42%). If you put the money elsewhere and it turns out you undershot the LTA, you've given up that gain.
This table shows the various outcomes from saving £1 of gross salary, depending on your income tax rate when you put the money in and the rate when you put it out.
This time I've ignored investment growth completely because as above, it just scales everything by the same amount (note that this isn't the case if you don't use a vehicle where the actual growth happens tax-free - e.g. you just invest money directly).
I've included Lifetime ISAs for completeness even though they don't apply to you.
Tax in | Tax out | Under LTA | Over LTA | ISA | LISA
40% | 20% | 85p | 60p | 60p | 75p
20% | 20% | 85p | 60p | 80p | 100p
40% | 40% | 70p | 45p | 60p | 75p
Finally, the lifetime allowance is now increasing with inflation, so you should be thinking about returns in excess of inflation when you project when you would exceed it. But of course, who knows what a future government might do.