It will take as long as the solicitors will take to agree who is to blame, then quantum. If those issues are settled early, then it can be within a year.
If cause and effect is agreed (the cause was professional
Negligence and the effect is a loss to you of £70k) then your sols will submit their offer, say £70k damages due to you plus your costs, the other side will come back with no, £30k and so on.
If you cannot agree, it goes to court. But no firm wants that.
You will have to prove she reasonably should have seen the risk and you reasonably weren't expected to see it for yourselves (mitigation). Has she accepted
Blame?
You won't get all your damages, there will be some give and take.
It will be, hands down, the most horrific time for your original conveyancing solicitor (is she a solicitor?) - that's not taking away from your loss at all, just don't think that it's passed to the PII guys and then it's all happy days, back to work. She'll lose sleep over it, but she's never gonna make that mistake again and you must be put right. It's why people use solicitors - we have outstanding PII cover. The public must never be worse off for using us. Hopefully her firm will support her, because we are all just human. I really hope your damages get covered - or nearly so. How much are we talking? Is it a retaining wall issue?