That's a difficult question to answer, because I think it boils down to policy preferences. Keeping that “debt servicing costs”, as we call them, lower as a proportion of tax revenues implies that decisions need to be made on areas of spending.
All of that is to say that there is no magical number or line in the sand where things get out of hand or become catastrophically expensive, so there's no clear point where things are deemed to be unviable or too expensive.
But it's true that if you compare the debt servicing costs with other government expenditures, they can seem to be very expensive and they can lead to difficult trade-offs if, for example, you have to spend more on debt servicing costs than you spend on transfers to persons and provinces, if we get there.
I'll stop there.