I'm not so sure. Wouldn't we expect more archaic forms of P312 and U106 in the East if that's truly where they formed? I'm not that familiar with placing the centres of diversity of those, but certainly L21 and U152 are centred in Europe.
I meant that if the mutations P312 or U106 appeared 5000-5500 years, it would have taken a few centuries before these lineages really takes off and started making a few percent of the PIE population. If the PIE population 5500 years ago was of 100,000 men (no idea, just an example), how long would you expect it would take before a new mutation, found in only one man, reaches 1% of the male population (1000 people) ?
Let's take the two theoretical examples to illustrate:
1) in a perfectly stable population where couples have two kids in average, one boy and one girl who lived long enough to procreate. Even if that man had twice more children than the average (2 boys instead of 1) and his sons, grandsons, great-grandsons, etc. always kept that rate of twice more sons than the PIE society's average, it would take this Y-DNA lineage 10 generations (about 250 years +- 50 years) to reach 1% of the population. Of course, after that, if they keep that pace the lineage can quickly become dominant. It only take 5 more generations to reach 33% of the population, and if unstopped, the lineage would reach 100% two generations later, i.e. at the 17th generation after the original ancestor. Things could go even faster if a chieftain/king's lineage with many wives started procreating 4, 5, 6... times more than the rest of the male population. The main drawback of this scenario is that a lineage of thousands or tens of thousands of people cannot have enough privilege over the rest of the population to keep having more children than average. This is usually limited only to the chieftain/king and some princes or high nobles, making at best 0.1% of the population.
2) in the same stable population, let's imagine that only the king's lineage produces more offspring than the rest of the population. This time, let's give the king a harem and let him have 10 times more sons than the average (10 instead of 1). However, only one of his sons will have the same procreation rate, while all other sons will have the average number of sons (1). The lineage therefore expands from 1 to 10, then from 10 to 19, from 19 to 28, from 28 to 37, etc. This is closer to the reality, but it also takes longer as only one man at a time has super-privilege. After 10 generations, there are only 100 men of the original lineage, or 0.1% of the population. The progression is fast but not exponential. It would take over 100 generations (2500 years +- 500 years) to reach 1% of the population at this rhythm.
However, whichever the scenario, things rarely, if ever, turn out that way for a single lineage. Diseases don't discriminate between royal/noble lineages and others. Wars always pruned the male population in ancient times. If the ancient Celts are any indication of the way of life of their PIE ancestors, the sons of chieftains were more likely to die at war as the warrior class was essentially the nobility, not the peasants (this was also true in India in Republican Rome, or in Medieval Europe). Furthermore, chieftains/kings get killed or deposed by other men, and the new dominant lineage undoes the progress of the first one.
The question is, when did R1b lineages passed the no-return point by becoming so overwhelmingly dominant that they always made up more than 50% of the male population in Western Europe ? Did it happen very quickly at the beginning of the Bronze Age, or was it a long process with plenty of ups and downs with other haplogroups ? We also cannot rule out that modern R1b lineages only started exceeding 50% of the population during the Iron Age. The Iron Age revolutionised warfare and made it possible to create huge armies that would overrun and destroy any neighbour stuck with sparse and expensive bronze weapons. This surely could have been a good time for the expansion of U152/S28, but also for J2 lineages in Italy and the Near East.
For all we know, P312 and U106 could have existed as a stagnating tiny minority of the PIE population in the Pontic Steppes for over a thousand years before the lineage takes off. If there were only a few men, or even a few dozen men carrying those mutations, and side branches got pruned regularly by wars and diseases, the STR variance would not have evolved much. There is no way to know that based on modern STR values. That's why I always prefer to overestimate a bit the age of a haplogroup rather than underestimate it.
I think that the STR method at best gives an approximation of the expansion time, after the real take off that makes a lineage soar exponentially for enough generations within a short time frame. Whether the mutation appeared 300 or 3000 years before that take off, there is no way to know without ancient DNA tests.