According to p.1 "API changes require discussion, use cases, etc. Code comes later.", I'm going to start discussion about benefits of proposed changes. 

Now com.google.common.cache.CacheBuilder contains such methods as
`refreshAfterWrite(long duration, TimeUnit unit)`
`expireAfterWrite(long duration, TimeUnit unit)`
`expireAfterAccess(long duration, TimeUnit unit)`
Parameters of methods listed below are used to define durations in time.
However, in Java 8 new class,  [java.time.Duration](https://docs.oracle.com/javase/8/docs/api/java/time/Duration.html), was presented. It can be used to save info about time durations in exactly one object. That is easier and more transparent in some real-world scenarios. 

The idea is to add overloaded methods:
`refreshAfterWrite(Duration duration)
expireAfterWrite(Duration duration)
expireAfterAccess(Duration duration)`

Proposed changes provide possibilities to simplify code:
`expireAfterWrite(Duration.ofHours(3))
// instead of
expireAfterWrite(3, TimeUnit.HOURS)
`
It makes easier to declare constant durations too:

`public static final Duration EXPIRES_IN = Duration.ofHours(1);
// some code after statement
expireAfterWrite(EXPIRES_IN)
`
instead of
`public static final long DURATION_LENGTH = 1;
 public static final TimeUnit DURATION_TIME_UNIT = TimeUnit.HOURS;
 // some code after statement
 expireAfterWrite(DURATION_LENGTH, DURATION_TIME_UNIT)
`

Also, it allows to execute some arifmetic operations like addition or subtraction, that can simplify program logic if multiple (and dependant) durations in different methods and/or in different caches are used. Just something like this (simplified synthetic example):
`
Duration expiresAfterAccessIn = Duration.ofMinutes(30);
CacheBuilder.newBuilder()
                    .expireAfterAccess(expiresAfterAccessIn)
                    .expireAfterWrite(expiresAfterAccessIn.plusHours(1))
                    .build();
`

This improvements do not lead to changing min java sdk version and do not break existing API.
checkState(keyEquivalence == null, "key equivalence was already set to %s", keyEquivalence);
    keyEquivalence = checkNotNull(equivalence);
    return this;
  }
  Equivalence<Object> getKeyEquivalence() {
    return MoreObjects.firstNonNull(keyEquivalence, getKeyStrength().defaultEquivalence());
  }
  /**
   * Sets a custom {@code Equivalence} strategy for comparing values.
   *
   * <p>By default, the cache uses {@link Equivalence#identity} to determine value equality when
   * {@link #weakValues} or {@link #softValues} is specified, and {@link Equivalence#equals()}
   * otherwise.
   *
   * @return this {@code CacheBuilder} instance (for chaining)
   */
  @GwtIncompatible 
  CacheBuilder<K, V> valueEquivalence(Equivalence<Object> equivalence) {
    checkState(
        valueEquivalence == null, "value equivalence was already set to %s", valueEquivalence);
    this.valueEquivalence = checkNotNull(equivalence);
    return this;
  }
  Equivalence<Object> getValueEquivalence() {
    return MoreObjects.firstNonNull(valueEquivalence, getValueStrength().defaultEquivalence());
  }
  /**
   * Sets the minimum total size for the internal hash tables. For example, if the initial capacity
   * is {@code 60}, and the concurrency level is {@code 8}, then eight segments are created, each
   * having a hash table of size eight. Providing a large enough estimate at construction time