How do you know when it's time to reassess your IT strategy?

You'll know. Regardless, I take a fresh look at it minimally after 2 years to ensure we're still aligned with the business strategy and guiding principles we have for using technology to deliver business outcomes.

When your budget starts to balloon, or you keep having to ask for more money to complete strategic initiatives.  If you are spending more than you planned for, it usually means that you have fallen behind the technology curve to such a degree that you are having to spend to catch up, or that your budget planning was incorrect.

I think 1-2 times a year at a minimum is appropriate. Throughout the year, there should be a feedback loop to see how your customers are being served and charter's being met. However, I think during a recession, an inflection point, or any disruption it is a high priority to assess how your IT strategy is meeting the business objectives. My experience with many customers & colleagues at the C-suite table is that the business is more empowered and enabled. Couple this trend with higher uptick in security, higher costs of operations, different needs for serving customers (remote) and a growing number of competing priorities, many IT departments target operating model should be assessed. Does our digital strategy align with growth? Do our portfolio & projects allow for customer experience? Are we able to draw insights from desegrated data to bring additional value to the business? There are so many options for the construct, practices and culture for an IT strategy, the assessment determines if the function is serving it's customers. 

IT Strategy should cover 3-5 years, I prefer 3.  It should be reviewed annually as a part of a deliberate and streamlined overview of the entire IT domain.  A conscious effort should be made to ask the questions like:

Is our long term goals "our Northstars" still valid?
Is there anything that has occurred that should trigger a change to strategy?

- If nothing external has triggered a change then -- Are you on track and if not, how can you get back on track. 
- If something external has triggered a change determine if minor or if it requires a revisit of the strategy.  

External items can be:

Major change in technology landscape -- cloud computing was one, Web 2.0 could be considered another, Identity management, the rise of Ransomware, distributed workforce. 

Major change in business landscape -- major change to business objectives, mergers, acquisitions or divestitures, change in company economics or change to the economic landscape as a whole. 

The IT Strategy has to be aligned with the Business Strategy, so should follow the same cadence. We also need to consider the ongoing changes in technology, regulatory, and business environment to realign the strategy accordingly, at least annually if not more frequently.