At this point, it's just too early to say. But imagine this timeline for Tomasino:
2020-21: last season of junior
2021-22: first taste of pros in Milwaukee
2022-23: maybe Preds, maybe Milwaukee
2023-24: maybe Preds, maybe Milwaukee
2024-25: first real chance he could be a top player for us if the previous two seasons he did well
In 2024-25 Johansen will be on the last year of his deal, and Turris' deal will have just expired, so that checks out. Duchene's deal ends in 2025-26. So I could see him getting a chance around that timeframe...
IF he's able to stick at center in the NHL... which so far as I've heard is not really likely. He probably has a much better chance of being a top-6 winger sooner.