I may be wrong, but I think I heard that 20 years is the maximum.
And that that time would be reduced for people who contribute/integrate to the country.
Which would be a rational proposal.
At the moment, it seems like you can get here, get someone pregnant (even before your case has been decided), and immediately claim a right to family life under the EHCR and then the courts block your deportation.
Perhaps though, would-be immigrants would be wise not to have children without thinking about the consequences - i.e. can you support them without recourse to benefits, and are you prepared to uproot them and move home again. This latter point is not that much different to people who (legally) move from UK to Australia, Germany, Canada, etc and then decide it's not the country for them (or lose their job there) and repatriate.
I think in this debate we always focus on the outliers.
If someone has been accepted here as a temporary refugee during a conflict, say, and does not support themselves or their family during that time, and does not look like they will do better in the future - why shouldn't they go back to their country of origin?
I think here on MN we'd say any man doing that was a cocklodger and tell the woman to kick him out rather than carry on supporting him.