The time for Sebastien Buemi's evaluation as a future Red Bull driver will be up at the end of the season.
Toro Rosso team boss Franz Tost told F1's official website that drivers should get a "maximum (of) three years" to show they have the potential to follow Sebastian Vettel in readiness for a top seat.
"Or let's put it this way: if you are not sure after three years whether he's going to make it, I would bluntly say forget him, period," said Tost.
Swiss Buemi, 22, made his F1 debut at the start of 2009 and is now considered the most likely Toro Rosso driver to make way for Red Bull's latest protege Daniel Ricciardo.
"I expect him (Ricciardo) to best (HRT teammate) Liuzzi after not more than three or four races," said Tost.
He admitted that both Buemi and his current teammate Jaime Alguersuari are "still deep in" the self-discovery phase.
Let's wait and see how the second half of the season works out
"They are moving forward ... so let's wait and see how the second half of the season works out. Then at the end of the year we will sit together with Red Bull to discuss who will sit in our cars in 2012."
Tost said he is looking forward to working with Red Bull's next youngsters including Ricciardo and Frenchman Jean-Eric Vergne.