How to forecast Share Repurchases ?

I'll give a few high level links as well just for some basic reference, but will dive into your question as well

In my experience, you should see or receive management guidance on share buyback amount per quarter or per year. Commonly the buyback plan was to signal confidence / give the market sentiment that management thinks they're being undervalued by the public markets. If the company has metrics such as FCF yield or some metric per share that investors pay close attention too, buybacks help boost those to be more attractive for investors that use those metrics for investment criteria or quarterly targets

 For modeling, use the disclosed buyback plan or guidance from management as well as the quarterly target / expected EPS and an assumed multiple for the share to trade at (ie 10x earnings). Then you can back into the # of shares to buyback in forecast quarters/years. Depending on the company, you may have to tweak what's inputs vs calculations for the share buyback schedule. If there's no guidance, you can look at the historical share buyback amount per quarter / year & assume similar amounts in future, following calculation logic mentioned above 

Share Buyback Rationale

Basic Share Buyback Modeling