Default value for server.servlet.session.timeout

30 minutes for spring-boot-2.1.8.

   <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.8.RELEASE</version>
        <relativePath />
    </parent>

I couldn't find the documentation, so the source code looks like this:

package org.springframework.boot.web.servlet.server;

public class Session {

	@DurationUnit(ChronoUnit.SECONDS)
	private Duration timeout = Duration.ofMinutes(30);