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.
* {@link Cache} interfaces. The {@code asMap} view (and its collection views) have <i>weakly
 * consistent iterators</i>. This means that they are safe for concurrent use, but if other threads
 * modify the cache after the iterator is created, it is undefined which of these changes, if any,
 * are reflected in that iterator. These iterators never throw {@link
 * ConcurrentModificationException}.
 *
 * <p><b>Note:</b> by default, the returned cache uses equality comparisons (the {@link
 * Object#equals equals} method) to determine equality for keys or values. However, if {@link
 * #weakKeys} was specified, the cache uses identity ({@code ==}) comparisons instead for keys.
 * Likewise, if {@link #weakValues} or {@link #softValues} was specified, the cache uses identity
 * comparisons for values.
 *
 * <p>Entries are automatically evicted from the cache when any of {@linkplain #maximumSize(long)
 * maximumSize}, {@linkplain #maximumWeight(long) maximumWeight}, {@linkplain #expireAfterWrite
 * expireAfterWrite}, {@linkplain #expireAfterAccess expireAfterAccess}, {@linkplain #weakKeys
 * weakKeys}, {@linkplain #weakValues weakValues}, or {@linkplain #softValues softValues} are
 * requested.
 *
 * <p>If {@linkplain #maximumSize(long) maximumSize} or {@linkplain #maximumWeight(long)
 * maximumWeight} is requested entries may be evicted on each cache modification.
 *
 * <p>If {@linkplain #expireAfterWrite expireAfterWrite} or {@linkplain #expireAfterAccess
 * expireAfterAccess} is requested entries may be evicted on each cache modification, on occasional
 * cache accesses, or on calls to {@link Cache#cleanUp}. Expired entries may be counted by {@link
 * Cache#size}, but will never be visible to read or write operations.
 *
 * <p>If {@linkplain #weakKeys weakKeys}, {@linkplain #weakValues weakValues}, or {@linkplain
 * #softValues softValues} are requested, it is possible for a key or value present in the cache to
 * be reclaimed by the garbage collector. Entries with reclaimed keys or values may be removed from
 * the cache on each cache modification, on occasional cache accesses, or on calls to {@link