According to this link, 10 term sheet negotiations leads to "5 agreements" and "3 full deals signed" (page 6). This model suggests a biotech drop rate of at least 50%.
It's a "big deal" in banking too. You rely on that to pay for DD, attorney drafts, etc. But it is not a "done deal." Roughly 20-25% of term sheets in banking are dropped. I have no idea what it is for biotech.