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As a general recommendation, growers should aim for 1/2” to 1” seed depth — even if dry — in early May and then consider going deeper to chase moisture in late May. In late May, soil temperature is consistently warm and seeding deeper to reach moisture should still provide fairly fast emergence. But know your drill. If you aim for 1.5”, are all openers in and around that depth? You want consistency.
To answer the headline question, going beyond 1.5” may be as deep as you want to go. If moisture is at 3”, for example, that is a long way for a small canola seedling to stretch. This increases exposure to seedling diseases (in time and surface area) and the longer time to emergence will reduce the protection period for flea beetle seed treatment. Energy required for emergence from that depth could also reduce seedling vigour.
A quick soil moisture test. In heavier soils, dig down to 1.5” depth and squeeze a ball of soil in your hand. Does it stick together? If yes, moisture in the seed zone might be higher than you realize.
Further reading:
Seeding into moisture: How low to go? (You’ll find lots of good considerations in this article.)
Questions about seeding into dry soils