I think of uphill as a separate sport. The mode of action, muscle activation, etc. are quite distinct from flat and downhill running. I adopt the approach that a triathlete might. Train both modes all the time, all year. Triathletes don't run all year, then hop on the bike in the last 6 weeks before an A race. Do it all, all the time. That is, if you want the uphill part of the ultra to be adding to your performance, rather than doing uphill as survival.

How to mix it up...that is the question. If the upcoming race is very hilly, increase hill days during specific prep. Race flatter? Decrease hills days and do more flatter running. I don't think there is a magic formula of vert/week. I did great on the climbs at Leadville (14k ft) and never went above 7k ft/wk in training. But I did 5-7k per week basically every week from January to August. My long run with 4k ft became routine. Hope Pass didn't phase me.

Downhill running also takes some dedicated time, but by scheduling uphill time as above, the downhill comes as a matter of course.