I kind of feel, and this is simply based on the personal observations of a few dozen parental leaves (including a few in my own family) that it's not so much the duration of time, but the ease back into work that is the important factor in reaching maximum balance for the family and company. I think 6 months off (26 weeks) is a good number, but spreadable over 18 months at the will of the parent. I feel this would make things much better than a cold-cut 6 months then back into the fire that a traditional 6 month leave would garner. I see a 3 months of full time off, followed with 2 days a week for 3 months, then 3 days for 2 more months before returning in full would be more beneficial to most companies and families to avoid the "that resource is gone for more than a quarter and needs to be replaced" conflict that happens during a 6 or 12 month "cold turkey" approach. I've also seen noticeable anxiety in mothers about returning to the workplace after a 12 month absence, compared to a 3 month.
I see the Canadian system being praised a lot in these comments, and in fact there are many laudable benefits to the system, but several acute shortcomings which are often overlooked when touting socialized parental leave. The system is firstly paid by deductions made to all salaries, which differs from an employer-paid system where the employer pays the leave. Under the Canadian system, if a company decides to pay an employee for the period, the government contributes that much less. This has a chilling effect on companies paying for leave, where almost all do not since there is no credit to employees working for a company which offers this, and individuals are still taxed the same on this portion. Additionally, it is a maximum of 12 months split between the father and mother, and depending on the duration and the % of father or mother, is a sliding scale between 50% and 75% of either your trailing salary for 6 months or a maximum of $60,000 per year, whichever is less. This makes it very difficult for a single breadwinner to support a family of 3 for a period of 1 year off of this benefit alone. In the majority of cases I'm familiar with, those individuals who took their 2 weeks and returned to work simply because the proposed benefit is not enough to pay the bills, thereby forfeiting tax benefits that they paid for and will continue to pay towards for the rest of their salaried life. And while you do stop contributing to this for the year once you pass $60,000 in pay for the year, there is no "exemption" if you don't use it and certainly no "payout" if you don't. In addition to this, you are taxed at your marginal tax % on any benefits you receive from this program, making it even less palatable for someone in a high tax bracket looking to take several months of leave. Someone in this tax bracket is usually working in a role that requires specialized knowledge or skills and are by that very nature harder to replace/restaff - even looking away from the impact to the company, taking that leave in many cases results in making some career sacrifices for that parent. A 3 month fully paid leave (to both parents) with no tax deduction would be preferable to the current system in almost all cases I've seen.