I don't believe there's anything in the Constitution that would empower a lead link to say "spend no more than half your time in this role" and expect that to be a hard limit that someone absolutely must adhere to, in the same way the lead link could if they had a budget to allocate and said "spend no more than $x in this role".
That's because "disposing of assets" (e.g. spending money) is limited by §2.1.3, which applies a hard and clear limit - and even Individual Action has to honor it, per §4.3.1. I wouldn't interpret allocating time/attention to a role as "disposing of assets", so there is no inherent restriction around it in the same way there is around spending cash or getting rid of organizational property.
However, the lead link can specify priorities and expect people will use their judgment on how to reasonably integrate them, per the duty of all circle members in §4.1.3(c). So the lead link could say "prioritize these two roles equally" and expect the person to integrate that prioritization and put a roughly similar amount of time into each role, at least on average. But that doesn't necessarily mean they won't at times put one role ahead of the other, or go beyond 50% to one role for awhile, because it's still up to the person to figure out how to incorporate that prioritization given their own local knowledge of their local context.
So, this is still a tool a Lead Link can use to achieve some intended balancing of time between roles: clarify the priorities, and expect people to figure out how to integrate them. But that's a different level of "hard and clear limit" than applies in the case of spending money (or disposing of assets of any kind), where it's not left to best judgment on how to integrate in a local context - it's simply a hard limit. (Though, you can always create a policy that enacts a hard limit for time allocation, if that's useful/needed to address tensions.)
Hope that helps!