<div id="session-timeout-warning" class="govuk-warning-text">
  <span class="govuk-warning-text__icon" aria-hidden="true">!</span>
  <strong class="govuk-warning-text__text">
    <span class="govuk-warning-text__assistive">Warning</span>
    <p>You must save your answers by {{session_timeout}}.</p>
    <p>
      If you don't your session will time out for security reasons and you’ll
      lose any information you've entered on this page.
    <p>
  </strong>
</div>

<div id="session-timeout-warning" class="govuk-warning-text">
  <span class="govuk-warning-text__icon" aria-hidden="true">!</span>
  <strong class="govuk-warning-text__text">
    <span class="govuk-warning-text__assistive">Warning</span>
    <p>You must save these answers by {{session_timeout}}.</p>
    <p>
      If you don’t save, your session will time out for security reasons.
      You’ll lose any information you’ve entered on this page.
    <p>
  </strong>
</div>

Change content of session timeout warning
Change content of session timeout warning

Updated the message with suggestions from @pocketstefan.
